Harry and Ron burst into the girl's bathroom. They had tried locking it in a random room where it would be unable to hurt anyone, but Hermione's shrill scream soon alerted them to the fact they had in fact accomplished the opposite of what they had set out to do.
There, they found a crying Hermione, for some reason, one of the Greengrass twins, and a giant troll looming over them.
"Confuse it!" Harry cried to Ron, throwing a tap that had fallen off of a sink at a wall.
The troll, bewildered, turned around to look at the source of the noise. Its stupid little eyes met Harry's, and it decided to switch targets to him.
Meanwhile, Ron had reached the opposite wall.
"Oy, pea-brain!" he yelled, throwing a metal pipe directly at the troll. Distracted, the troll turned to Ron, giving Harry the time for a tactical repositioning.
"Come on, run, run!" Harry yelled at Hermione, pulling at her in a bid to get her away from danger. She shook her head, unable to speak, and pointed at Oleandra's body.
It's at this point that something incredible happened. As the Troll approached Ron who was stuck in a corner with no means of escape, Harry did something monumentally stupid. He jumped up and latched around the Troll's neck, planning to stick his wand in its eye, or something. Normally, this wouldn't have fazed it even a little, but just as Harry took to the air, ice crept under the troll's feet. At the same time, Ron's mind emptied in fright, and cast the only spell fresh in his memory, the Levitation charm!
"Wingardium Leviosa!"
The club was yanked out of the troll's hand.
And Oleandra, who had taken the blow under the full protection of Elhaz, was in better shape than her appearance let on. While all this was happening, she had taken Hermione's advice to heart and dipped all five of the fingers on her right hand into the water that pooled on the floor and murmured, "Isaz…"
And the water turned to ice.
All three of these occurrences happened simultaneously, leading to an incredible turn of events! Still frozen in fright, Hermione watched in amazement as Harry leapt onto the troll's back, making it lose its balance, slip on the ice and land on its back. As Harry rolled away safely, Ron relinquished control over his Levitation charm, making the club fall onto the troll's head, with predictable results.
Had the club fallen on its head while it was still standing, it would simply have been knocked out and woken the next morning feeling slightly grumpier than usual, but instead the club had an extra troll's length of time to fall on the monster's head, making its head explode in a gruesome manner quite akin to an overripe watermelon bursting, sending flecks of bone, blood and brain matter flying everywhere.
Still in shock, all four of them looked at each other's bloodstained appearances, not quite believing what had just happened. As they stood there wordlessly, they heard footsteps approaching.
Professor McGonagall, Snape and Quirrell burst into the room in quick succession. Quirrell took one look at the troll, then gaped in amazement, completely forgetting to put on his scaredy-cat persona.
Oleandra gulped. Professor McGonagall was positively livid. This wasn't going to be pretty.
"What on earth were you thinking of?" spat Professor McGonagall. "You're lucky you weren't killed. Why aren't you in your dormitory?"
Oleandra raised her hand, as if she were in class.
"Um, what's going on?" she asked as innocently as possible. "I never made it to the feast, why did we have to fight against a troll?"
Professor McGonagall couldn't explain why a troll had appeared in the girls' bathroom, so she pushed back with another question.
"Why weren't you at the feast with the others?" she asked.
Oleandra revealed that she had come here to return a book to Granger.
"Miss Granger, why were you here, then?" Professor McGonagall asked Granger.
"I went looking for the troll because I — I thought I could deal with it on my own — you know, because I've read all about them," she answered with a voice full of remorse. "And Harry and Ron, they came looking for me."
Oleandra, Potter and Weasley tried their best not stare at Granger with wide eyes.
Oleandra was surprised, though not as much as the two others. Little Miss Goody Two-Shoes, lying to a teacher to her face?
"If it hadn't been for them, I'd be dead now. Oleandra took a hit for me, you can see the mark on the wall where she was sent flying. Harry jumped on it and brought it down with a wrestling move, with the help of Oleandra's ice magic."
She pointed at the still frozen floor.
"Then, Ron smashed its head in with its own club. They didn't have time to come and fetch anyone. It was about to finish me off when they arrived."
Granger, in all her sneakiness, had purposefully drawn the Professor's attention to the fact that Oleandra had used strange magic. She was taking advantage of this situation to learn more about it!
"Well — in that case . . ." said Professor McGonagall, staring at the four of them, "Miss Granger, you foolish girl, how could you think of tackling a mountain troll on your own?"
Granger hung her head in shame. The other children were speechless. What had happened to the stickler for the rules they knew?
"Miss Granger, five points will be taken from Gryffindor for this," said Professor McGonagall. "I'm very disappointed in you. If you're not hurt at all, you'd better get off to Gryffindor Tower. Students are finishing the feast in their Houses."
Granger left despondently.
Professor Snape cleared his throat.
"Miss Greengrass, Miss Granger mentioned you were hit?"
"I'm fine," Oleandra said lightly. "The club didn't actually hit me you see; the wind pressure from the club blew me off my feet, that's all."
"Then what about that Greengrass-shaped hole in the wall? Surely that's not wind pressure, hm?"
"Umm…"
And then Professor Snape escorted her to the Hospital Wing, where she was diagnosed with seven fractured ribs, a light case of whiplash and a slight concussion.
And then he awarded her with fifty points for her major role in the defeat of the troll!
Although Oleandra would never know about it since it would happen after she left, Professor Quirrell slipped on the ice she had created and banged the back of his head on the hard ground.