Repost of my earlier suggestion from temp forum. Creatures of pure energy, both benevolent and malicious. Immune to most conventional weapons, they should be real pain to handle, requiring to change your tactics and equipment to fight them. Weapons that can damage them can be annoyingly ineffective in most other situations, being slow to flire, prone to misses, weak, et cetera. Basically, it should be a group of weapons no one ever uses, "because they suck". Turns out, not always. Various "elementals" are probably in related category. And they are cool bunch, but they aren't what I am talking about. These energy creatures are closer to fusion of pixies and ghosts. Actually, they could be somewhat close in appearance to these phantoms from first unsuccesful Final Fantasy movie. Yeah, it's been bad, but phantoms looked very impressive. Oh, "phantom" is not bad name, by the way. Threat level: high.