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."

Rin told her, and Shelly nodded. She was fast enough to barely make it to safety as the boss charged toward her, and Rin let out a sigh of relief.

It would take a few minutes for the ice to melt and the bombs to go off in the boss’s stomach. Hopefully, being blown up from the inside would be enough to keep it down.

"Everyone, be on the defensive and keep your distance from the boss. We don’t know when it will explode. Don’t let it aggro on a single person."

Rin reminded everyone, and they nodded.

The monster was focused on Shelly, so Albus started attacking its vulnerable spots from afar. This caused the monster to change targets and head toward Albus. But as soon as it got close, Serena threw ice-made projectiles at it.

The boss changed directions once more, and it was Shelly’s turn to distract it.

It was a highly effective strategy that was starting to drive the boss mad. So the boss made a decision.

Instead of going after the ones who were attacking it, the boss decided to go after the ones who were not doing anything. And the one closest to it was Teressa.

Teressa was not prepared for the sudden assault, but her instincts immediately reacted as soon as the boss came close.

"Don’t come here!"

She yelled as she used her powers to hold the boss in place. But the boss struggled against her control, putting a strain on her powers.

[You have exceeded your limit. Please stop using your powers, or your body will start falling apart.]

Teressa noticed the message, but she could not move. If she did, then the monster would be able to move again and she would be dead.

’In the end, I am alone here once more. Am I going to die? Then what am I working so hard for? Can’t I drop my powers and let the boss take me? I am sure that would hurt less than whatever I am feeling right now.’

Teressa’s arms were hurting, and her legs were feeling shaky. She was having a hard time even standing up.

And yet, she knew that she could not give up. This would be the end for her if she dropped her powers.

She did not want the pain, but she did not want to die either. It was a contradiction that kept her working hard.

But hard work could only take her so far.

[You have suffered a fatal injury. Your corruption level has increased by 5%.]

The message startled Teressa, but not as much as someone grabbing her waist and running away with her in his arms.

She looked down in shock, only to see Rin helping her out.

"We seriously need to look for a way to increase your skill so that you can move around while using it. You are far too vulnerable while fighting."

Rin complained, but it did not seem like he was upset with Teressa. She felt relief fill her body as Rin carried her to safety.

The boss noticed that she was gone a little late, but it immediately looked for her. It ignored the other attacks and immediately came after them.

But it was too late for it. The ice in its stomach had finally melted, and the bombs inside started to go off.

The monster did not cartoonishly explode like monsters did in movies. Instead, its whole body jolted a few times before it collapsed.

Serena used that opportunity to cover the monster in ice, and both Shelly and Albus landed final attacks on the snake’s neck and underbelly, finally taking it down.

It had not been a long fight, but it certainly was one where they came close to losing their lives.

"D-Did we succeed? Was that all we needed to do?"

Amelia asked as she came out of her hiding spot and started checking on the others. She looked relieved to know that the fight was finally over.

But... was it finally over? This place was still present, and the notification for clearing the task had not appeared.

[Oh my! That was very observant of you, Rookie! You are right. This task is not over yet. The final boss behind the curtains has yet to be revealed.]

The system confirmed what Rin already knew.

Clap

The sound of clapping was loud, and it echoed in Rin’s ears. He looked toward the source of the sound and noticed a well-dressed figure heading their way.

The figure was headless, but somehow it still looked handsome. That contradiction was enough to make Rin tense.

"You all did such a good job defeating this monster. However, there is one last task I need you all to do for me. Hmm, your expressions tell me that you do not know who I am. Well, let me introduce myself—I am the owner of this dungeon and also the one who called you here. You can call me Mr. H."

The headless body was somehow capable of speech.

Rin had no idea how that could happen or what it meant, but his instincts told him that this person was bad news.

He gripped his weapon to attack the person, but Mr. H waved his hand and everyone except Rin froze in place.

"Don’t try and attack me because it will be useless... hmm, why is my authority not working on you?"

The boss asked, and Rin did not know how to answer—mostly because he had no idea either.

Why did the boss’s ability not work on him?