Shadow Ball, and dumb luck.
The other day I was battling on PO, on the first turn my Alakazam got knocked down ot 1 HP and I made some bad predicitons and ended up behind my opponent. After some lucky hits, I evened the odds, and it ended up being Tyranitar and 1HP Alakazam against his Latias. Latias knew recover, and spent almost 20 turns recovering and using 4 or 5 Calm Minds before KOing Tyranitar. With nothing else to do, I sent out Alakazam, congragulated my opponent, and clicked Shadow Ball. I had given up on winning, so imagine my suprise when Shadow Ball gets a critical hit, and KOs Latias from about 75% health with 4-5 Clam Minds under her belt.