>>24715003>>24715038More reliable solution:
Specs ditto becomes Mega Mewtwo Y, then gets skill swapped Adaptability because your opponent's Porygon-Z really wanted Pressure.
Then, he switches to Chansey because he realized he fucked up. This gives you the chance to use Psyshock, which has a 25% chance of killing this thing.
252+ SpA Choice Specs Adaptability Mega Mewtwo Y Psyshock vs. 4 HP / 252 Def Eviolite Chansey: 564-664 (87.8 - 103.4%) -- 25% chance to OHKO