So for this achievement if you don't have a friend on hand to help you I recommend the second level of act 1. This was the earliest level where the floor for player 2 didn't give way on me. Other than that be wary of the archers that spawn. One of them will hit your player 2 character. But there is enough hearts that pop up in the level for you to keep at full health. You will also earn the Twice the Fun achievement if you haven't earned it already.