Mirror Mail didn't work? Surprising. And I think you should have realised by now brute force doesn't work well on Fafnir.

An alternative strategy (the one a friend of mine used) involves having the tank equip weapons and armour to maximise evasion and then cast Decoy on that character. Fafnir focusses on that character only and as long as they're close, it'll waste its time trying to hit him and missing over and over.