Managed to down him. Interesting story this time:
First time I get to B15, I clear about 75% of the floor (up to around 1.9 million) and find another mysterious portal. I take it since I haven't skipped any of them to that point, and I figure why not.
So I take it and there's no fucking exit. At all. Luckily, the game makes an auto-save now when you descend a floor, so I load that and start B15 all over again.
This time, I clear maybe 15% of the floor, and the first damn door I have to kick down after running out of lockpicks, that fucker is right on the other side of. Literally one tile from him now, which is real bad. I go to leyline teleport the heck outta dodge and I click one square out of line, which is an invalid move (you can only go in straight lines with the leyline teleport) giving him a free turn, which he uses to hit me with thunder for about 70 damage. I wasn't sure if Dredmor could see through invisibility or not, so I gambled on dumping a Wyrmling in his face, thinking a healing item would do no good if he got a second turn with me as a target, which worked and he smacked it with his staff. Had he decided to just bolt me again, it would have been donezo, but instead I was able to make my teleport and invis him off me with a potion. After a few health potions, I moved back into that area leading with Wyrmling casts about 10 tiles out front, and once I hooked him on the line this time I blew his ass to kingdom come.
Unfortunately it cost me about 120,000 points since I wasn't able to go back for the rest of the level, but I'll take it. Stay free, Dredmor.
The new floors aren't too different overall, except for the diggle gods - if you happen to find one of their shrines, you can pray to it and gain their buff permanently (or until you choose a different diggle diety). I only had 3 in my game; War (which I didn't take, figured it was for melee), Digging (which was a bunch of pierce damage, health, armor and a couple other stats) and Death (which was an amazing grip of resists, 15 to just about everything, but prevented you from having health or mana regen).
Not sure if what I hit was a known bug, but I would warn everyone to ignore mysterious portals at least on B15. I did notice that the dungeon graphics were not like the other portals; it was using the tiles from B10 rather than the plain grey tiles. Be warned!