Hmm, Lavawalk should do the trick. This is the final section of the game, so you don't need to cross until you've explored everywhere and done everything else.....
.... i.e., freed Kerah with holy water. Then you should see, instead of a door, two bars in the doorway - only then should you try and cross. You need the two champion keys to open the bars up. If Lavawalk still doesn't work, also try multiple Toughen spells to raise your HP. There's an amulet off in the lava, but it only provides 50% fire protection.
- John
