After you have beaten any level on Easy, Hard and Bro mode you unlock Pug mode for that level, if you choose pug mode you can only pick between Edgar and Maya, they both have 1 heart and can't use any special attacks or defense so whatever level you choose you need to be extremely careful as one hit from anything will kill you and there are no checkpoints so you need to do it in one go. I personally would suggest using the Hurry Up! level to get this achievement as it is the shortest and probably the easiest level in the game.