I've been playing sorcerer and haven't had to melee anything so far. Everything has been killable with spells, even those that say "immune to magic." For example, cave viper says immune to magic yet I've killed them with fireball, though it takes a long time, or with chain lightning, which 1-2shots them.

I think immunity to magic refers to spells that don't use an element, like fire and lightning.

Yeah it's a bit confusing. There are three types of magical attacks: fire, lightning and magic. Hence, "magic" is not the generic term including all spells, but only a smaller part of them.
"Magic" type spells are: blood star, bone spirit, flash and berserk (hellfire only). A monster immune to magic is only immune to those four spells.

Interesting. Thanks for clarifying, I was wondering about this as well.