Ok this dude has a Shiny Mewtwo with the ability No Guard and It has 3 OHKO Moves And Psychic, All of its stats are at EXACTLY 400, HOW DO I WIN??
Here's a way for it to work. either let your pokemon hold a focus sash ( I've tested this, it works) then use the move endeavor. Then hit mewtwo with sucker punch. or some priority move. That will take him down.
Quick claw+ destiny bond= win.