Incorporeal. Not solid. As in, you can walk through them if you so desire. The point of spirits is to aid allies with area effects or damage and hinder opponents, not stand in the way like a braindead nightclub doorman.
^And that's only if it's still possible to kill them aswell, they are annoying when you get stuck on them and stuff... But still it's annoying when they're wackin ya one and you want them dead for it
To Clarify, he does not mean make them invulnerable. He means make them a non-solid object, so that characters, heros, and henchman can walk through them without being obstructed.
I don't like being blocked by them and it has happened before. I want to be able to do the same thing with minions, please? I know that defies physics, but think about what else thing game does. Allied minions only, of course. You can get blocked by enemy ones.
You can attack them but still pass throught them...
Unless you do something like : they can only receive elemental or magic damage, I see an inconstency. (then again, that would make spirits more powerful... a good thing)