What an absolute healing overload, but I did it!
Switching over to Warfarin's S2 to boost Exu's damage worked really well and as soon as the boss was gone, I could throw everything towards the other side.
And of course the challenge mode is doing it without ANY medics... not looking forward to that. Buuut that's a headache for another time. For now I'm content to have beaten chapter 6.