Phantom Wound gives the illusion your victim's HP is reduced to 5d4 but does not literally set them there. Due to this, it should be made saveless.
1. It's a harmless spell; it does no damage at all.
2. Counters exist via spellcraft and also by memorization of how it appears when casting it.
3. It's trading 1 round of action for another round of action except the enemy's round of action has no DC.
It's honestly one of the most ingenious spells about but the DC seriously hinders it use.