Quote Originally Posted by Eldariel View Post
I believe I already mentioned Tenser's + Haste [needs to be recast a couple of times by your Simulacrum but it has enough slots for this to not be an issue] from a level 13 Wizard with 16 Dex and a +1 Longbow. It does enough damage to overcome her Regen and kill her in about 4* minutes (150' range). If you happen to have Elven Accuracy (e.g. you're a Bladesinger or an Evoker-type interested in Scorching Ray + Spirit Shroud), you can do 45 DPR which translates to 15 damage a turn counting her regeneration. 585 [615-30 - she drops to 0 before regen]/15 = 39 rounds so about 4 minutes, give or take. If you have an 18+ Dex (e.g. from Magic Jar: this adventure features plenty of good targets, e.g. the Dragonsoul Cultists which give you +3d6 on an attack with advantage and 18 Dex) body, you don't need Elven Accuracy - 18 Dex and +1 weapon would do 40ish DPR which amounts to 6 minutes to kill her . . .
Do you think the calculation would get a lot riskier with events causing you to have to move about the arena? I mean, like, imagine that everyone agrees on this as the plan in and out of character- including the DM. Imagine you have a board-game challenge scenario where you have to keep moving to other cardinal directions from the forcecage as Tiamat's avatar slowly transforms the arena into a hellscape of obscuring atmospheric effects, raging wildfires fifty feet wide, literal pools of acid, icy terrain, and clouds of gas piling up on the arena. Nothing she could do would make her obscured from all directions, but she could definitely force you to have to move around her a lot if the DM ruled that she can produce effective Fog Clouds with her breath weapons. ......

.......not to mention her spending all her actions on the Dodge action and her breath weapons on her legendary actions out of turn.