His regen stacks up to three times, and is not based off the characters on the field (but it DOES proc once at the start of the turn of every character. In other words - get rid of non-damage summons when killing Thork). It is based off his artifacts which are laid around the battlefield. If you mouse over them, you'll see " Artifact - Thork" These are the ones you want to break to reduce his regeneration rate. I think the lowest you can get it is to 5%, by breaking 2 artifacts? Not entirely sure there, but it's definitely reduced by breaking artifacts.
The artifacts have 100 HP, so they're fairly easy to break - but! An even easier way to break them is to simply end your turn on them. Ending your turn on an artifact will break them instantly.
Good luck and happy Wakfu-ing ^^
Ah, I should mention - enemies ending their turns on artifacts will not break them. An ally has to do it.
Also...Bwork Dungeon has this relaly annoying bug where you are absolutely unable to do anything for a turn. I'm not sure what causes it.
It's a green artifact (don't confuse it with yellow one). When you (or maybe even bwork, didn't test it) end your turn on top of it (standing on the same cell), Thork loses one stack of wild regeneration. Plain and simple.
Yeah, that bug is still there and it's pretty fierce. I ran two times by single character and had to restart client in fight once in first fight and twice in second. Bless Ankama for combat reconnect, but they still need to fix range bug, though.