"Trojans are invulnerable, except for a large crystal on their backs, and will rapidly turn to face their target. When fighting them with a group, whichever player the Trojan is currently tracking (usually the knight who has dealt it the most damage) should keep it facing away from the other players so they can attack. If fighting one solo, it is necessary to bait it into attacking and strike it from behind before it recovers, or use bombs, circling so the Trojan will turn its back to them."
- http://wiki.spiralknights.com/Trojan
I think that if Phantoms donated their little purple tracking thingies to the Trojans, everything would make much more sense.
Really, if everyone just moves a little, you'll know who's the trojan target.
Phantoms are sillier because you can't tell where they're facing when they are constantly evading attacks and turning into orbs.