Killing those Monsters was tricky. The moment those Monsters sense their life will be in danger, they will immediately hide themselves inside the pond and that poisonous pond was also protecting them from their predator. Because of this, you cannot find those Monsters other than all these poisonous ponds.
But as you know that Sam did not even give those monsters any chance to react. He already used a sun ray to instantly attack those monsters with that. That attack was so fast that even those monsters did not get the chance to react and it already killed many frogs at the same time.
But in the next second those remaining frogs become very alert and they instantly hide themselves inside the pond. Not only that, at this time purple gas also started coming out from the pond, and that purple gas began to cover the whole pond.