This question has already been asked in general form here, but was closed as too broad: "How to deal with broken combos?". I’m going to make an effort to make the scenario I’m asking about more specific to keep my question open.
The spell sickening radiance (XGtE, p. 164), cast just before a wall of force (through a readied action or something similar) can trap and kill many solo Big Bad Evil Guys (BBEGs), with just two casters. This combo is particularly lethal because of the levels of exhaustion the spell gives, but could work even with other persistent damage spells. Sickening radiance is a spell that creates a persistent area of effect for 10 minutes that deals radiant damage and gives exhaustion on failed saves.
The combo works like this: wall of force (WoF) caster readies their action, with the trigger "cast WoF on the BBEG when my friend casts sickening radiance (SR)". SR caster’s turn comes, and they cast SR, centered on the BBEG. WoF gets cast on the BBEG as a sphere, trapping the BBEG within the wall of force for 10 minutes while they get exhausted to death.
What actions can I, as a DM, take to make a fight with a BBEG not as trivial, while still rewarding the two spellcasters’ ingenuity?
I classify "rewarding the two spellcasters’ ingenuity" as at least forcing the BBEG to spend a limited resource to deal with the problem. So a BBEG having disintegrate is one option for what I can do, but giving the BBEG a teleport as an action (or legendary action) is not.