You can kill the mobs resurrected in the fire area over and over.
I'll be very surprised if Larian doesn't fix that little lava-idol-resurrected-skeleton-EXP bug/exploit soon, thanks to Xendran's mad abuse (check out his screenshots in another thread). So no more level 20 characters in Cyseal. Thanks for ruining it for the rest of us man.
But seriously, as long as the alleged hard level cap at 20 (and if this does actually exist: why, Larian? why?...; you don't seem to be the type of developer to take such a cheap way out of implementing a properly balanced game) isn't something completionist players will always reach before the endgame, I'd tolerate it. Otherwise, hard level caps usually kill the potential for further party development, and make completing any additional optional content (for EXP, anyway) kind of pointless.