edit: which, i realize, is what you did originally. disregard, i'm an idiot :|
edit 2: might be part of a text-based tutorial, depending on how much story you plan on putting in the game. maybe a note from a character suggesting they don't like the light? might help dense folk like me pick up on the environmental cue on the first encounteranother idea might be to have the first encounter with light sensitive enemies signal it by forcing the player into a combat engagement.
i can picture it now, the first time encountering a light sensitive enemy there's a defective light that flickers on and off, enraging the enemy during the light and allowing it back to rest in the dark.
more like an environmental trap/simple puzzle than an enemy on the first encounter?