Best bet with Rumphy is to fry him from a distance with Lightning or Zap (Stone Magic, which he's not very resistent to). He respawns every time you return to Kerah's statue, so you can run back and forth between the statue and Rumphy's tomb to get mondo XP...great character trainer! He does have a "petrification" percentage (something like 2% chance) so it rarely happens, but if you do try to take him on "one-on-one" make sure you save!
I've always encountered Banshees in the Crypt, but then I've always played on Hard/Frequent Spawn so I tend to get a LOT of encounters. (I'm an XP hog).
-Sazerac