Yep, as someone else pointed out the bard gets to treat and d20 for Persuasion/Deception as a 10 and then add their modifier.
Still thinking about the boss fight and the other actions surrounding it and the greater context though, lots to think about.
They can use persuasion in Phases 1 & 2 to lower their health too but I think you might be right with the 3rd phase 'this seems to be useless, lets kill!' mindset may well set in.