With my solo mage, the demon in the Planar Sphere was an extreme challenge. What I eventually did was use the Wand of Monster Summoning to distract it, cast Lower Resistance at least once, and Skull Trap twice. I hate those Death Gaze's, don't you? Anyway, that strategy works well in a number of situations.
"Ah, home, sweet home... wait a minute... I don't live here." - Jan Jansen