It's weird how some spells are resistible and others aren't. There totally needs to be monsters that are virtually immune to physical attacks and then others who are totally immune to magical ones (and probably a few that have almost both).
Right now, do fireballs hit a monster taking up 4 squares 4 times? AoE damage spells are probably the only ones I could think that viably could/should hit multiple times, but other spells like wither, mass curse, etc. are silly effecting in more than once. It must be targeting the squares instead of the units themselves.