This one's one of the easier level based challenges, and the level is pretty small so even if you mistakenly flame breath one of the monkeys before realizing this was the trophy, backing out and going back in will be a slap on the wrist.
There are seven monkeys scattered throughout this level - they're the only enemies that you'll see hanging from trees, and shoot ranged attacks at you as you approach. Due to their height you won't be able to hit them with a normal charge, but if you run up on them, jump and then press square you'll have no trouble hitting them.
The monkeys are even divided across the three sections of the level - the "main floor" where you come in, the treetop buildings, and finally over the cliffside that you reach with the spring gate after killing 20 enemies on the level. Just make sure to ram each of the seven and it should pop.