@ Cuttler: How are any of my examples Metagaming? Metagaming is using out-of-game knoledge in game without the PC/NPC having justifiable access to it.
A reasonably smart enemy could easily figure most of my points out after one round of "HOLY F**K THIS GUYS BLOOD KILLS US IF WE MAKE HIM BLEED!!!"
If PCs encountered an enemy who had this ability, they would also change their tactics accordingly.
- "look, the blood only goes so far, if we stay clear, we're cool"
- "Hey, if the wounds don't bleed, there's no blood to kill us"
(use bludgeoning or spells)
- "Hey, how about we try to make his blood hit his friends, archers, concentrate fire"
Are all reasonable thoughts/decisions for an NPC in battle. I would maybe put in some perception or intelligence checks for the enemies, but that's about it.
Of course all of this only comes into play if none of the enemies succeeded on their spellcraft check to identify his spell being cast and just tells his allies "Don't cut him, his blood kills" as a 6 word free action.
If you have an actual Big Bad and not a series of random encounters, the intel about the players tactics should filter back to him eventually anyway.
If the players only meet one group after the next of mindless and/or stupid enemies with no intel gathering skills, then this spell is going to kick major a**.