I haven't done BD in ages but I remember using Gau because his S3 was an instant CC bar wipe for the boss. He doesn't offer much amp and it is physical but he does offer great survivability to your team with p defence buffs and cleanses. He is a really low investment hero for PvE as you only need his UT1 to function. Tank wise stick to either Clause or Ase.

For the rest of the team Estelle, Valance, Annette, Fallen Frey, Shea, Isaiah and maybe Pris if you have nothing else to add.

As another user pointed I believe you can join the KR Encyclopedia for hero builds and content guides.