Party Hearty

The one Score Attack achievement that needs co-op: every participant must score at least 200,000 points in a single mission. The standard trick is a co-op run with a 2x Skull enabled, taking your time to clear everything so both players comfortably pass 200k - which in the same sitting can also bag The Tortoise, Up For A Challenge and Drop A Quarter. See the linked walkthrough for the go-to route. Just make sure both players are actively scoring, not spectating.