Hmm, I presume that a frozen heart should be somewhat uncomfortable to an 100% cold immune seraph as well. Maybe serpahs should not be 100% cold immune, or maybe FrozenHeart should affect cold immune beings, but should not work on heartless beings...
(Maybe lifeless should be immune then, but freezing the mechanics of a mechanical man could be nasty too?)
(I think I already said somewhere else already that I do not like 100% immunities happening too often anyway...
)