idealy, your healer should be able to cast multi-heal (or omni heal, depending on how much mana you have) and recover any damage you took. so having too much defence wont really help much... even more so because the boss has several spells available too, which defence cant help with at all.
it might be annoying, but i do recommend you resynth your team and level them up again. that will give them some extra stats and skill points anyway. and that never hurts. the balhib, slimestack, demon-at-arms would be a good team to fight him with, you just need to fine-tune their skills a bit more