wait til new game +, then in the new game + file, go fight ozzy flea and slash in the secret room in the bend of time. equip a forget-me-not-pot on somebody and have that person get the finishing blow on each one of them. this will let you use those monsters in the grand slam, but only for one round, and i suggest you use it for the hardest.

for the other two, i suggest finding harder animals like in the terra tower or something like that, and finish them off with the person with the forget-me-not-pot. the first two shouldn't be THAT hard...