Cast into Darkness
Pacifism handles the attacker; this handles the blocker. The choice to leave toughness untouched and strip two power plus the ability to block tells you exactly what kind of game this Aura was built to win: not the stalemate where you neutralize a threat, but the race where you want their creature stuck out of the way while your own board keeps swinging. The -2/-0 is a continuous static reduction, in effect the entire time the Aura is attached, but it only points one direction: it never touches the body's survivability, only its offensive output. A creature with two or less power gets blanked as an attacker entirely, and the can't-block clause takes it off defense at the same time, so the enchanted creature stops mattering to the damage math on either side. It will not save you from a large attacker, since the toughness stays put and the shrunken body can still swing back with whatever power it has left. That is why it lives on the offensive rather than in reactive decks buying time. Two mana buys a surgical tempo dent, not a removal spell: against an idle creature it does nothing worth noting, against no creatures it is dead in hand, and against a single oversized threat it is only half an answer. The reward for accepting all that narrowness is the price and the speed.
