You must have been hiding behind pillar too long - Easy mistake to make I guess.
What setup are you using? I've done that quest 2 times, and the easiest method I have tried with is using Range gear with Addy bolts (e), having a Spirit Kyatt summoned with about 300 Ambush scrolls in your invent (you won't use all, but just to be safe), some Super Restores (only 2-3) and the rest of your inventory filled with Sara Brews.
You aren't bringing a proper ratio of Brews to Restores because you are only relying on the special attack from your bolts, which has the same accuracy and hit regardless of your Range level.
Range Nomad, spam your special attack for your Spirit Kyatt, and sip brews to heal yourself when necessary.
When he gets down to melee form, make sure you protect melee (it doesn't completely negate damage, but it does slow down his attacks by a considerable amount) and stay within melee distance so he doesn't heal.
The whole time, just make sure everytime you aren't eating or hiding behind the pillar from his twins/that one powerful attack, you are attacking him and SPAMMING your kyatt attack.