It's really not that hard.
First Buldoze turned the field into a cave, 2 more Buldozes makes the cave collapse, KOing every active pokemon. Dittoceus can be as strong as he wants, he still won't survive a cave on his head.
Or you can use a pokemon with sturdy/focus sash and Destiny Bond, or Perish Song, or FEAR rattata. Or a Curse Ghost and wait him out.
Or just bring your A game, paralyse him, or toxic or something, spam status moves till he's weak and than crush him.