You have to reach EXACTLY zero dust bunnies to get this trophy. Every time you summon something, you use three bunnies. If you wind up with one or two bunnies left, and you don't have enough to summon more, head into outer space and pick up some gift boxes -- each has five. If you have one bunny left, get one box, then summon two things. If you have two bunnies left, get two boxes, then summon four things. Either way, once you wind up at zero bunnies, the trophy will ding!