1) Read the epic advancement rules in the DMG.
2) 20th or below? I have just shown how 4 30th lvl damage-specialised wizards cannot kill the T. with the best (by far) damaging spell in the game...
3) Other spell will be less effective the way you have this monster statted - check them all out. If you don't agree - name the spell(s).
4) As a GM I would make mincemeat out of a party of that composition with this boss, frankly.
Perhaps not intended, but your post seems oddly hostile. Anyway, here are my responses:
1) I have read the DMG and there are not rules for extra
levels (perhaps you should re-read the DMG or my post again). There are rules for advancement beyond lvl 20, but it based on epic boons (with a variant to take a feat or ASI instead). In fact, the DMG states: "
An epic boon is a special power available only to 20th level characters." They are specifically for 20th level characters, not 21 or higher. Also, you mentioned that your theoretical 30th level wizard had nine 9th level spell slots; however, the epic boons state: "
Unless a boon says otherwise a character can't gain it more than once." And the boon of high magic does not specifically mentioned that it can be used more than once. So, by RAW there are no 30th level PCs, and a wizard cannot get nine (9) 9th level spell slots.
2) Yes 20th level, the official highest level of the game. Again, meteor swarm is the highest damage spell, but it is a poor choice against big T. (he is immune to fire and has resistance to bludgeoning). MS does a max of 35 avg. damage while a 9th lvl disintegrate does 106 avg. damage.
3) see my response to #2 above. Finger of Death would also do more damage, as would a 9th level lightning bolt.
4) I think anyone who plays big T. intelligently would be able to wipe out any 4 PC group. However, if a DM plays big T correctly IMO (as a beast with an intelligence of 4) it becomes much more interesting. The fighters have at least AC 26 and +20 to hit and a 152 DPR (76 with big T's resistances) for 4 rounds. If they can gain advantage it would make sense to take the -5 to hit for +10 damage to add another 80 (40) damage to the DPR. That is approx. 2,784 (1,392) hp damage over 4 rounds (not including any damage from the cleric). So, with 3 fighters so kitted and help from a good cleric to keep the up and buffed. It seems possible, though unlikely.
EDIT: +476 (238) DPR if we include critical hits and vorpal swords or just +98 (49) w/out vorpal sword
EDIT 2: AC 28 with ring of protection & defensive fighting style
EDIT 3: that is a grand total of 3,260 (1,630) damage over 4 rounds. So they need to hit about 60% of the time to make that work (in 4 rounds). With taking the -5 to hit (for +10 damage) these guys hit AC 25 about 50% of the time. So, if the cleric can keep these guys up, they just might be able to pull it off in 5 or 6. That would be some epic battle though!
EDIT 4: Not RAW, but since its size is 60x60 I would probably rule that all rays from prismatic spray would hit big T. That makes it a very effective spell since big T is potentially taking 105 damage and is restrained and blinded.
In fact, finding a way to blind big T is possibly enough to turn the fight as it severely limits its ability to hit the fighters and increases the odds of them hitting, making GWM much more effective. I think I may need to give it a type of blindness resistance! Maybe a nictitating membrane for advantage against effects that blind.
EDIT 5: If they all take a potion of speed, we get: AC 30 and +262 (131) damage over 4 rounds for a grand total of 3,522 (1,761) damage. Assuming advantage (big T is blinded or some other source) gives approx. +3 to hit bonus that gives our fighters a 65% hit rate more than the 57% rate they now need to finish him off in 4 rounds. If the cleric can spend all its time buffing and healing, they have a pretty good chance now. An no epic boons needed.
EDIT 6: It takes Big T about 2 rounds to take down one of these fighters (if it is not blinded and no crits). So the cleric just needs to keep all 3 fighters up for 3-4 rounds (no easy task) and they should have it. This makes think big T is a little weaker than I had hoped, but you do need a pretty crazy build to take it down.