I don't have a definite idea of the NPC's levels, just based on the DM I'm guessing they are built as characters of around ECL 13, which makes them more-or-less CR 13. And we already beat the tar out of the verminlord once, he managed to get away and apparently ally with (what we thought were) a completely different set of enemies in order to ambush us. My DM isn't batshit-painthuffing-crazy like yours, Kevin, but he is a malicious sonofabitch, apparently.