This reminds me of Anachron Ninja in the Archive : it can't take damage, but dies when targeted by a spell. I'm not exactly sure how infinity health would be coded, but if it can't, you might want to change the effect to can't take damage(like a passive ability), and make it lose this ability when targeted by a spell.
Thematically it fits. You could possibly say the Cherub represents a child, "grew up" after he was targeted from a spell, before that he was innocently shielded from the cruetly of the world.