Sam is carefully observing Monsters as he wants to make sure that Monster comes here naturally. As the monster is trapped inside the barrier, it is unable to do anything.
Sometime later,
Sam was just about to kill that Monster when he saw something interesting present inside the body of that monster. It is a very small thing that normally you would not be able to see.
By chance, Sam was able to notice that thing. It is a small coin-like thing present near the core of the monster.
Now, naturally, this makes him very curious. The thing is that that coin-like thing did not get dissolved by the body fluid of that monster.
So that thing is made with some kind of material which would not dissolve that quickly.
Sam decided to kill that Monster, but he needed to make sure that he did not destroy that coin-like thing. After opening a small part of the barrier he immediately created a water bullet that directly hit the core of that monster.
