ROTDS beta testing

Battling the Lava monster in Thamasa;

I can't for the life of me hurt this thing! I killed him the first time with Osmose because I can't do enough damage to offset his 2000 regeneration per tick. I do an average of 500 damage per attack and 1000 with by bigger hits. (Terra was able to do 4000 morphed/splash, but once his reflect goes up...)

My bigger problem is fighting the second battle with Krosis. His Inverse is nice except when reflect comes up and lasts too long for his regeneration to bring him back to full health.
Shadow can do maybe 1000 damage with Shuriken's but dies so easily that Leo is constantly reviving him and is otherwise on healing duty.
I'm wondering if there's a strategy I'm missing here. I'm sure poison heals him too so I tried using Aqualin on the boss. I don't know of any confuse abilities I could use to make the bomblettes explode on the boss for me (which woulda been kinda cool).
Am I just too underpowered to break his regeneration? It doesn't seem like that's the answer seeing as most bosses have had a tangible strategy to squirrel around but this time I'm just stuck once he starts to protect/reflect and gets his regen on. I'd even go as far as to Rasp/Osmose him to death again but the second battle doesn't have magic users.

What do?
:Average level is 25.