Here is the passage with grammar and word-usage edits applied while keeping your style intact:
The boss attacked Shelly, but she did not move. She kept standing in place since this was what Rin wanted from her.
He had not told her what to do, but she could guess what he wanted based on why he had chosen her and the help he had asked for.
The monster was close enough, but Shelly was not paying attention to it. She was paying attention to Rin, who was throwing the last of his explosives between Shelly and the boss.
"Serena, freeze the explosives. We need the boss to swallow them."
Rin yelled, and Serena immediately froze the explosives in ice.
The boss noticed this, but instead of avoiding the chunk of ice, it decided to swallow it whole. This seemed to be a habit Rin had noticed beforehand, so he felt relieved when his plan worked.
"Shelly, get out of the boss's way now."
