Killing Tiamat inside a Forcecage

[Eldariel]

Not here to shut you down- I honestly want to hear how you'd take the practical response to openly force-caging this thing and thinking that you'd be safe. What's your best 121ft combat option?

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 (and 180 arrows, which is indeed a lot but between the party you probably have that much - the opportunity cost for carrying lots of arrows is pretty minor). But even a standard Dex 16 Wizard 13 would need only minor help from literally any source to get the job done (Magic Weapon spell or Elemental Weapon or such if there are no magic bows in the party and someone capable of doing any damage whatsoever).

Other options, well, if you have a slot left (say, another level 13 Wizard in the party or you're level 15 [which the adventure sets as a plausible level for this point]) is to use magic. Cloud of Daggers only lasts 1 minute so it's impractical but the perennial companion of Forcecage/Wall of Force, Wall of Light, cast out of 8th level slot does do 7d8 per turn for 31,5 which adds 1,5 damage a turn. You need your Simulacrum to add a spell on top of it (another Wall of Light works as does Cloud of Daggers) for a total of 63 damage a turn which kills her in a bit under two minutes (18 rounds on average) and lasts for 10 minutes. Indeed, two Cloud of Daggers of course suffice too; 80 damage a turn from Cloud of Daggers VIII x2 does her in in 12 turns - not quite enough but if you have literally any source of damage on top of that (a source of 8,5 damage a turn would suffice). But of course, you're more likely to have Wall of Light than Cloud of Daggers anyways if you're using Forcecage since it's the better spell to go with it (for, among others, the reasons mentioned here).

If you have a Cleric, there's Summon Celestial VI or VIII, which works great. Summon Undead VI or VIII would work too. 17 DPR for Celestial 6 (or 35 if you can give it Advantage with Concealment, by e.g. casting Silent Image of Darkness or Greater Invisibility [you're outside True Seeing range] or whatever to let it attack from Concealment giving it advantage) or 13/30 for Summon Undead 6, and 20/45 for Summon Undead 8 and 22/49 for Summon Celestial 8. It goes without saying that it doesn't take a lot of these to kill her. Both spells last an hour so one casting is enough as long as they can do enough damage. On a party level, that shouldn't be a problem. For a single character, the best option depends entirely on your build but there are plenty of tools that work here.

She literally has divine word. That's WOTC letting you know that this Divine Avatar may not be an exarch rolling around with epic divine power accessible at will but she's still undeniably a divine powerhouse- instead of cleric casting she uses a more carnal manifestation summoning raw elemental strength over positive or negative energy drawn from followers or what have you. Their 7th level cloud of daggers is actually sick, but you still have legendary resistances to count in. Still, you're getting in the danger zone even casting Cloud of Daggers with Distant Spell, as 120ft is still dangerously close. Their use of Haste is very smart, but Haste will make you lose a round before the hour is up. Tiamat would have to survive until your haste runs out, then bust a move. Forcecage too has a 100ft reach, and the caster would have to escape her breath weapons without suffering loss of concentration. What I love about this is it's still a fight, even with her chained in place. Because you gotta get close enough to start it, then get out of range without dropping any of your limited 7+ level magics.

Forcecage doesn't have Concentration. The DoTs do but you don't need to be in range of her breaths at the end of your turn. Legendary Resistance doesn't matter since these DoTs are no-save damage. Well, casting Cloud of Daggers is risky but it lasts a short enough duration anyways: Wall of Light is better (as per usual).