Eloquence Bards get to treat any roll on certain charisma skills as a 10, whenever they roll less than a 10. So with expertise and high charisma they can guarantee a roll of 22 or higher in those skills related to eloquent speech.
Usually the DM can set difficulties or even decide if a roll is called for at all, but this module has a chance for diplomacy, and the corresponding DC baked in.
I agree, though, that the correct answer is “Let the eloquence bard do their thing!” The chance to talk the boss down comes in “phases” for a reason. As long as you still have to survive Phase 1 and Phase 2 before you can suggest surrender in Phase 3, you’ll still have an interesting encounter.
And there is always the possibility that by Phase 3, your bard player won’t even think about using diplomacy once the fighting has already been going for a bit.