The game does not have online matchmaking, so you will need to play with invited friends (up to 3 others) or couch coop with two or more controllers. If you are playing this game regularly with others through all three difficulties, you should get this through natural play.
There is also a large hill on the Pumpkin Pastures level, where you and your friend can run off the hill and take fall damage until they go down. Revive them and repeat grinding for the achievement.
Here is a playlist of guides for this game
https://www.youtube.com/playlist?list=PL-HeW6XkKdUKsFThxuvCp...