Wow, it's been a while since I've played that. Great game though. Anyways... Sometimes you just need to go to other areas and level grind a bit. I always keep at least 1 wavemaster with me for healing purposes, and a bunch of health and revives just in case. Don't forget when you break the data seal or whatever to do your Data Drain or else he'll just stay invincible. I personally generally use Blackrose and Mistrel (or Marely or whatever her name was). Also don't forget that most scrolls or whatever they are don't work on an infected monster, but will after you break the data seal thing.
Oh and a little note on the level grinding thing, don't forget if you fight through an entire dungeon to the end, then leave, and come back, the monsters and all that stay dead, so if you can't beat this boss yet, and leave and come back, you just run straight to the boss, you don't have to beat the entire dungeon all over again.
