Angering peaceful monsters by casting an innocuous spell like light is very annoying, so split the spell up into a “light area” spell, which does not damage anything unless it’s a highly light-sensitive monster like a gremlin, and a “light blast” or “flash” attack spell, which may or may not light the area permanently and is designed to blind monsters.

There should be a way to tell what the acquired resistances of a monster are. This could be by zapping a wand of probing at it, by breaking a potion of enlightenment on it, feedback gained from a stethoscope, or (less likely) a new spell exclusively for pet diagnostics.

Note that it may not be considered “fair” for any of these methods to work on pets, but not work to tell the player’s resistances when they do it to themselves.