Cheers!
If you have any room, avoidance is key. They have a time limit of 20 seconds, so just stay away. You can use crates to block them pretty well, or you could use greater teleport to quickly escape to other parts of the room. That may be needed in this case, depending on the room.
Another way to go is to try and fight them, and for that I guess it depends on your confines. If its long and narrow then launch meteor could do massive piercing damage. Just don't miss. Ice cross is also good, but you probably can't get all the fire in one burst. Leafy whip would actually be one powerful ally here. Heck, even creeping death, if you can launch it and hide behind it, could work well if it's upgraded enough to stay behind.
Personally I prefer to run. I'd start with the crates, etc. When the overlord switches attack patterns, then worry about damaging him.
Good luck!
