These things stay alive by feeding off of energy. They tend to stay near areas of high radiation. They are usualy neutral unless you attack them or are using a tool that uses energy. Shooting them with any type of energy based tool will actualy heal them instead of hurting them. They can only be killed with solids, like guns, or other non-energy based melee weapons.
So, light energy (Electromagentic radiation)? Why not make them attracted to torches, but they create illusions? They wouldn't be able to effect anything directly (Or be effected), but they could make it look like there were blocks in locations where they arn't, make blocks look different, change what labels you observe, and generally change what stuff looks like around them. They avoid sunlight, and only target the player if they're visible via the player's light source. You could go without artificial light (The light they radiate doesn't count) or you could use infra red, night vision, etc. I'd keep the 'found in high radiation areas' part, though. The illusions could be used to prevent players from finding stuff, unless they lure the energy beasts away...
Now from what your saying you shouldn't be using guns, PURE ENERGY (Light) bullets would just fly through! You could use unconventional techniques at first. Lead them to a energy collector (pulls it out of the air) or use some energy collecting device you get free electricity and you defeat the enemy. You could then upgrade to special anti-energy weapons, like bullets that pull energy from the air while they go. Nice idea though.
