GMforPowergamers
Legend
I dont have my books with me atm, but I think my point stands without it. Bearing that in mind:
Let's say the city guards are, in fact, using the Guard template. Assuming 1% of the people in a city of 25,000 are guards (which is a lot), and the rest are commoners, that's a force of 250 troops. Give them whatever bows they can find (assume light crossbows with range 320 for ease of calculations), and they'll have +3 to hit (+1 Dex + prof), against the Tarrasque's AC 25. Using the group combat rules from the DMG, each individual would require a 22 to hit... Since a mob requiring 19 to hit only hits once every 10 attacks, and a mob requiring a 20 to hit only hits once every 20 attacks, let's extrapolate that so that a 21 would require 30 enemies, and a 22 would require 40 attacks to hit once.
That gives us, of this force of 250 guards, 6 hits per round against big T. Factoring in their disadvantage for long range, and we can assume that, with their odds of 1 in 400 to hit, that outside of the 80ft range, nothing would hit accurately enough to damage the Tarrasque. Within the 80ft range, they'd have 1 turn to damage big T before it reaches the walls. Once it reaches the walls, it's game over for the city: the gates would be demolished, the walls obliterated, and soon the rest of the city would follow.
So how much damage would those archers do? Saying they get 2 turns of attacks, one before and one after big T gets into the walls. Hell, let's even be nice and say that they manage to avoid being killed by the wall being taken out from under them and all succeed on their saves, so the full contingent of 250 is firing. The guards would manage 12 attacks for 1d8+1 each, for a total of 66 damage. But wait! The Tarrasque resists damage from non-magic weapons, so they only do half that! Altogether, the Tarrasque takes 33 points of damage, the city is destroyed, and everybody dies. Good job guards, you sure showed him who's boss...
I could not find cross bow or bow stats on guards... I found knight and vetrian both have heavy cross bow...
vet:
H X-bow Ranged Attack: +3 to hit, range 100/400 ft.(1d10) piercing damage.
and
Knight and thug both
H X-bow Ranged Attack: +2 to hit, range 100/400 ft (1d10) piercing damage.
(((I can't belive there aren't Archer stats....but anyway)))
Big T :
AC 25 Hit Points 676 Speed 40ft.
can double move 80... so covering 400ft in 5 rounds...
now DPR isn't really my thing, but I will try. need a 20 to hit but 20 will crit... so 1/20 by 2d10 (11) is .55 so how many shots at .55 does it take to kill the most powerful monster in the MM, 676/.55= 1,227.2727272727 well I will just round that up to 1,230 attack rolls... how long does it take the Bit moveing 80ft (witch it shouldn't be shouldn't it be tearing things up) to go from 400ft to melee...5 rounds... so 1230/5=246 archers over 5 rounds...
If we add any PC help in here at all... or you know anything over a CR of 3 (and one of those stats is at CR 1/2) you will notice a problem...
now the longbow (((really no archer stats))) is less damage 1d8 vs 1d10 but 1.5x the range...
the idea of a 5th level mage doing anything to THE MOST POWERFUL MONSTER IN THE MM is crazy... and they are obviously some oversights...
why don't you need magic to hit the only CR30 in the book?
Why did they take away the regen?
Why did they take away needing to wish it dead...