Whether the Dash action increases the Tarrasque's movement for its legendary actions too is a matter of ruling. I always assumed it inc free a asked your speed for the round, period; hence, a dashing tarrasque getting 40' out of its legendary movement, not 20'. Anyway.
I decided to reuse an old 2e character who never saw much play, instead of Tomas; I like Tomas and want him to live. This guy was a tool and I'm okay with him being eaten.
Khelendros was a compilation of the character of same name from Dragonlance being cool and amazing, blue dragons in general being amazing, and the thought of a wizard who commanded dragons through respect, fear, and force of intellect was amazing. So I rolled him up using Munchkinry of the Standard Arts and am immediately angry at this guy for such amazing rolls; three 17s, two 15s and a 14. What a waste!
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Khelendros, the Evoker
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Variant Human Wizard (Evocation specialty) 6, HP: 39, AC: 15 (Mage armor) 20 (shield spell), Speed: 30 (fly 60), Saves: Intelligence +8, Wisdom +5
Strength 15
Dexterity 15
Constitution 18
Intelligence 20
Wisdom 15
Charisma 17
Skills:Arcana, Investigation, Perception, Athletics, Survival (Fallen Scion background)
Tools: Navigator's tools
Feats: Elemental Adept (electric), +2 Int
Gear: Ornate Robes & Cloak, Noble's outfit, survivalist, arcane focus (wizard's staff), component pouch, charmed breastplate (rune of alacrity grants dash as a bonus action once a day).
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The fight itself
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So we have this magic user haggling in a city for components and access to a library, when there is a furor as people flee the false safety of their towering buildings for the streets to run; the Tarrasque is coming! The tarrasque is a fifty foot monstrosity and the exact sort of trouble guards dream up being the heroic figure who alerts the town of, so there's no need for close range. The monster is taking its time, and a good distance away.
Khelendros rolls for initiative preemptively (12) and handily beats the tarrasque's own (6). Fair fights and white rooms are for suckers who aren't wizards, so he first scales the outer walls, thirty feet in height, and then casts fly on himself, leaving two remaining 3rd level spell slots.
The tarrasque simply rampages closer; it is now 200' from the city walls. Whatever brush or thatch-roof cottages (TROGDOOOORRR! *ahem* sorry) litter the area are chump change to the beast and any humans have long since fled before the wave of wild animals, panicked, running for their lives.
ROUND 2
Top of the round, Khelendros activates his rune of alacrity* and regards the monster; fifty feet tall, half again as long... he rolls well enough on his intelligence (arcana) check to recognize the primordial beast of the end, the Tarrasque. He knows it shrugs off directed attack magic and resists everything else. His signature lightning is almost useless. His higher spells should be saved to simply keep him alive; acid splash it is! Dash (bonus action) and then kicking off the wall at speed, Khelendros shoots a full 60 feet upward, and then flies out 120* towards the monster. He is now 90' off the ground and 80' away from the tarrasque, which puts him at 90' range.
For its part, the Tarrasque catastrophically fails to notice this vivid blue missile (perception: 4) until it enters the range of its blindsense. Once The wizard does the, Big T decides to eat the birdy. Legendary Movement at the end of the wizard's turn closes the distance to 60' horizontally, and it continues the run, intent on jumping and swatting the thing to the ground.
Quick math check - with a strength of 30, the tarrasque can jump a maximum of 13' up (high jump) and there's no check involved. That rounds to about 15, giving the creature a 'height' of sixty five feet for combat purposes, and a combat reach of 85' – just shy of our airborne wizard.
But wait! A creature jumping can also reach up half its height, for such purposes as grabbing at ledges and such. This means the tarrasque can grasp at an additional 25-35 feet depending on how you read this in relation to it being a long creature. That's just enough to touch the wizard, if not to attack him.
So the Tarrasque tries to grab the wizard***. The Tarrasque rolls a natural 20 on its grapple effort and, surprised, Khelendros doesn't even have time to reflexively use his Shield spell. Attempting to resists, Khelendros rolls his own athletics check (17) and while valiant, isn't enough to escape. Khel's player wants to ague about that extra five feet of reach, and the DM points out that the end of high jump allows an (Athletics) Strength check to exceed the normal height limits, and we could always allow the 20 to be on the jump, letting big T make an actual, damaging attack roll...
So the Tarrasque pulls the wizard downward by the boot, and then spits him out on the ground... except bolstered by his magic, Khelendros doesn't fall but hovers. He now sits almost nose to nose with the boost, in what cinematics would make into a story beat so we can appreciate how terrifying this is... cue frightful presence, which Khelendros fails with a resound (3) on the die. He is now 40' above ground, within melee reach of the Tarrasque and
ROUND 3
Khelendros, frightened almost out of his mind, makes a mad dash for the skies (movement) shooting up to a solid 100' above the earth, and turns to unleash his vitriol on the thing. As he begins to speed upward, the Tarrasque retaliates with an opportune bite attack (24) which Khelendros manages to barely skirt by casting his Shield spell. Close call!
Once khelendros hits altitude, he lets loose a globule of smoking acid (11 damage; DC 16) which strikes the still-snapping beast in the nostril (dex save 8). The Tarrasque is surprised.
On its own turn, it tries another running leap (29) but cannot cover quite that much distance; at an extra fifty feet the wizard is just too high up. Forget surprised; the Tarrasque is now peeved. It finishes its leap by landing and sort of trudging forward on its own momentum. This puts it 50' below and 40' away from Khelendros; the hypotenuse being 65', the Tarrasque is now out of range****.
ROUND 4
Khelendros is keenly aware of his predicament, and that his fear will put what the Akashic Records say should be an easy win into a slugfest on the monster's terms. He grapples with this for what seems an eternity, but is in reality eight additional rounds. Thinking furiously, the wizard allows his turn to pass.
The Tarrasque is not satisfied. It attempts once again to run and leap to snap up this gnat, and once again fails; the DM waives the athletics roll saying Khelendros is simply too high up unless something special happens.
ROUND 5
Khelendros is still thinking furiously, and still failing his saving throws. He cannot really do anything. He cannot move closer to the beast. He cannot attempt to flee the beast, either; its sinuous, shark-like circling means it may trap him as a matter of happenstance in such a way he cannot escape before his spell expires. He cannot ready himself to splash more acid on the beast when it gets closer; not only might the DM say the Tarrasque falls back to earth, finishing its movement before he can use his reaction*****, but holding a spell at the ready requires concentration, and staying airborne is his only life line. He cannot simply fly up and away, because he will be unable to move far enough to lose sight of the thing (altitude extending your visual range and all). He curses the day he read Treantmonk's guide and listened only to "always pick up the Perception skill".
Tarrasque tries one more circle, running, jumping, and snapping futilely, before just meandering to the city to consume it.
ROUND 12
Khelendros finally manages to shake off his terror, maybe because of the indignity of just hanging there, maybe because watching the City become a semi-smoking rubble heap puts his own fear into perspective. Probably because he finally rolled high enough with disadvantage to shake off the condition.
The Tarrasque is only approximately three hundred feet away, as the town wisely unleashed its entire draft-animal and ranch animal supply out into the field where the beasts ran and then froze in terror, and got et. Khelendros spends his turn dashing toward the Tarrasque, maintaining altitude at 100' above ground level. He closes and is now 180' distant.
The Tarrasque, occupied by an easy meal, notices the wizard but pays him no mind. Another twelve horses die to its hunger.
ROUND 13
Khelendros is past the two-minute mark. His fly spell will only hold for another eight, or approximately 79 rounds. Normally an eternity, but in a war of attrition, who knows?
Khelendros moves closer. He is now merely 40' away horizontally, once again just shy of range.
The Tarrasque devours six more horses and a cow. Khelendros thinks the DM is likely goading him by having an animalistic beast seem nonchalant at his presence.
ROUND 14
Khelendros is now ready to act. He moves moves the 40' necessary to hover over the beast and splash it, and he does for (4) damage. The Tarrasque's 0 dex bonus is really going to sink it, here.
The Tarrasque, annoyed, attempts and fails to leap and bite at the wizard.
ROUND 15
Khelendros lobs another blob of acid at the Tarrasque, which shrugs off the magical fluid (17).
The Tarrasque consumes two cows and a horse. It picks up one each cow and horse in its claws, and moves away 40'.
ROUND 16
Khelendros, faster than the Tarrasque, flies directly over its head and once again pigeon-bombs the beast, which avoids it with aplomb (19).
The Tarrasque begins to understand there's a minimum distance the bug needs to be able to sting. It runs away... and turns to lob a cow.
Lobbing a cow is an improvised action. It uses the range increments of the giant's boulder but at disadvantage. The effective distance between the Tarrasque and Khelendros for this throw is 50' up and 40' across, or 65'. The Tarrasque is already firing at disadvantage though, so that subtle difference doesn't really matter.
The Tarrasque manages an amazing natural 20, and Khelendros invokes Shield in response, hoping. No such luck, as the disadvantaged die is enough for a (25), just hitting his armor. The group eventually compromises and settles on improvised damage but with strength mod (24; concentration DC 12), which is almost enough to end the match. Khelendros manages to overcome the shock of being struck by a heifer and maintains concentration (22).
ROUND 17
The cattle finally bolt, their minute of utter terror finally expiring, leaving the Tarrasque with only a horse to throw. It kills it with a squeeze of scything claws and hefts the horse's body.
Khelendros reasons that this is probably the dramatic crux; if the Tarrasque cannot hit with its remaining projectile, he can probably convince the DM to condense the "fight" into sixty-some rolls to see how much damage he does. If he survives that horse he has this Tarrasque thing on lock. So he moves above the beast, and spits acid at it, which singes its hide (6 damage).
The Tarrasque flees 60 feet and turns, lobbing the horse (19), which is handily deflected by yet another Shield. Khelendros is down to a single 1st level slot, and will need to burn 2nd level spells soon.
ROUND 18
Khelendros continues his campaign of closing and acid-etching his legacy in the Tarrasque's hide, and the Tarrasque continues sloughing off any hits the wizard almost lands.
On its turn, Tarrasque uses the dash action and runs. It uses a legendary move to get further away.
ROUND 19
It's been three minutes.
Khelendros continues following the Tarrasque but fails to close the distance; at 50' above and 40' away, the hypotenuse (and his attack range) is sitting at that annoying 65' mark. He retcons a bit after doing the math and uses a Dash to get closer; still no attack but better than losing ground. Khelendros is now sitting above the Tarrasque.
The DM smiles, following this with a legendary movement... toward the city walls******. This puts the creature 20' away, but next to a thirty foot wall! The wizard swears a blue streak at not paying attention to direction and terrain. The Tarrasque uses that legendary movement as part of a running start, clambers onto the stone wall, turns, and kicks off, spraying stones and busted timber all over the city interior. The long jump is easily cleared and adds another seven feet to the Tarrasque's effective height; a fifty foot monster, springing from a thirty foot wall, clearing seven additional feet in its arc (rounded to ten via successful "Athletics check to jump higher" (29), to the table's dismay) is a height of 90' with a minimum reach of 10.
The Tarrasque attacks. As it's teeth close in on the mighty evoker (31) even his final, desperate Shield fails to protect him from what's coming. Whatever remains as the jaws close are quickly dealt with by the resulting tearing of claws (each attack auto-hitting and causing two failed death saves).
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*: not a canonical item, but the sort of "useless" trinket that shows up in games by about fifth level to encourage players to do wacky things.
**: the rules do not strictly allow a double-move; this is what the dash action is for. Using an action to move, rather than perform an action, is the sort of 3e/4e holdover I find logical, effective, and not unbalancing except in cases where magic items or rogue abilities give you dash as a bonus action. As the only change is how close the city walls would be (and I don't want Big T tripping over rough terrain) inn letting it slide.
***: this is the sort of DM I am and have played with for the last twenty years; where many people argued in 3e that the rules must be read as hard limits, I read a framework. Climbing creatures, leasing giants, shoving monsters onto oil slicks and using the encumbrance rules on moving them, all fair game. Without this style of play, this scenario would be much different.
****: Yes, I use geometry for three-dimensional combat. Always have, always will. The book is silent on this issue, but a strictly standard reading would still the creature at 10 + 8 squares away, beyond the 12 square reach of the spell. I could be convinced to extend the range as the spell drops, but with advantage on the save, though.
******: while a sort of jerk move, not unwarranted. Losing track of terrain when focused on angles and flight is something that's happened to every caster or ranged flier I've ever gamed with. It would only work once, and further tests to see if his was a statistical fluke won't account for it, but outside of a white room, terrain exists, and the Tarrasque is at least as smart as my cat. That kind of cunning would definitely lead a prey animal into compromising its safety zone.
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My synopsis is that, yes, a single wizard can solo the tarrasque. The logistical issue is strictly about being able to do enough damage to destroy the thing, before all your available slots for Fly are used up. A wizard 6 can't guarantee that; a Drow or warforged warlock/sorcerer can. I'm playing one now who could pull it off, due to some minor chicanery.
The tarrasque is at least as cunning as a house cat however, so it will run, throw things, use cover, create cover, burrow, etc. As it needs to. If there's any terrain anything all, it will utilize it. Not out of intelligence but out of animal necessity.
