Using the formation

Sam moved on to the next group and later the next one.

All he did was bait them with shadows of the Nagas. He didn't form any form of contract with the Naga shadows, he just summoned them. It would take at most the energy of a nascent stage cultivator's attack to summon them.

Since the contract is not here, he can do whatever he wants.

So, at every position, after he found any Nagas, he would summon the shadow and insert a grenade inside before leaving the shadow near another group.

It is as if Sam is on a lucky streak, as the next five groups clearly pressed the grenades without exception, at least they are little clever than the rest as only a single person would press the grenade a little farther than the group, but it is still useful to restrict the movements of half the group easily.