The reason bards do low damage in this fight is because they spend half of it in ballad mode. It's a very healer intensive fight. You need 1700 group damage to hit the fight comfortable. Your dpses must all be doing 300+.

P.S. Our bards each rotate using ballad for me and still manage to hit a bit above 300. They both have an unweathered rosenbogan though and are geared well enough..so.. I wouldn't hold that up to your standards, But our monk alone does like 430. X_X All our doses have unweathered weapon so they hit well above the 350 mark… but minimum for each person should be 300 even with weathered weaps otherwise you'll struggle.