The only way you can really do it is to kill each of your party members when he disappears. It's very useful to know Life2 and be prepared for a long fight, because it takes a while.

Basically, whenever he disappears, he'll possess one of your members. It's your job to figure out which one, and you have to kill each one until they die. If it's the right member, Wrexsoul will reappear.

I know he's resistant to some magic, but I can't remember which ones. The little creatures in front of him can't ever be killed, they just come back to life after a couple of turns. Basically, hit Wrexsoul with whatever you've got when he's out in the open, and you should kill him eventually.