Much like any magical creature that grew to over eight feet in height or length, Trolls were remarkably resilient to magical attacks. Offensive spells and curses seemed only to glance off their tough hides, and so the general consensus amongst the magical population was simply to get out of the way when faced with a charging Troll.
However, if one truly wished to get rid of a Troll, whether by repelling it or killing it, there were a few avenues open to those brave enough to attempt it. According to Professor Lockhart's book on the subject, Travels with Trolls, one first had to determine whether one was dealing with a Mountain Troll, Forest Troll, or River Troll; in descending order of strength and brutality, and in ascending order of cleverness.
Mountain Trolls were known to be strong enough (and dim enough) to attack Giants, though it always ended very badly for them. River Trolls were clever enough to learn the art of ambush; they would lurk beneath bridges and pop out whenever anyone attempted to cross, gargling some riddle, and while the poor traveller tried to decipher their words through their thick accents, the River Troll would yank off their heads while they were distracted.
A well-performed Charm could easily pacify a Mountain Troll, because its brain was scarcely larger than a walnut, and a well-aimed Conjunctivitis Curse could send a River Troll packing when it paused in front of you, trying to remember the words to its 'riddle' (assuming one was generous enough to call it that). As for Forest Trolls, they were as stupid as they were strong, so either method worked just as well.
Of course, there was a third avenue available to those brave enough to try it. Much like any creature that grew to over eight feet in height or length, Trolls were vulnerable to humanity's greatest weapon… sharpened sticks.
The woolly mammoth could certainly attest to their efficacy!
…
Oleandra thrust her hand in the air as she hovered over the crowd of screaming Muggles and Muggle-Born.
"Gungnir!" she cried. "To me!"
Oleandra waited for the familiar sonic boom and thunderclap that would usually ring out as the spear broke the sound barrier in its rush back to her, but after ten seconds, it was becoming increasingly clear that it was not coming at all. Evidently, the Death Eaters must have sealed it away, since she had left it lying around after using it to blast through half of St Mungo's.
The Trolls were streaming into the clearing now, almost upon the stragglers at the edge of the crowd. Julianne was pushing her way through the throng, having all but abandoned any attempt at crowd control in favour of intercepting the Trolls, but despite standing over seven feet tall, she was dwarfed by the sea of Trolls, each measuring between ten and twelve feet tall. She didn't stand a ghost of a chance.
What now?
Although his books were as enlightening as they were entertaining, Professor Lockhart had never said anything about facing more than three Trolls at once. And Julianne was carrying Tonks's baby; Oleandra refused to let her come to any harm.
The biggest, meanest Troll at the head of the pack raised its thick wooden club, readying itself to bring it down on Julianne, who had leapt over the last stragglers and was raising her arms in a futile attempt to block it…
"Flipendo!" Oleandra cried, whipping out her wand.
The club slipped from the Troll's grasp and flipped backwards, smashing into its face. Dazed and bleeding copiously from its nose, the Troll stood there dumbly as Julianne launched an uppercut at its chin, sending it toppling heavily backwards like a felled tree into its oncoming comrades.
Trolls were resilient to magic, that much was true… but the environment around them was not. Remembering how she had bested a Troll in her first year at Hogwarts, Oleandra unhooked Hufflepuff's cup from her waist and blasted the Trolls with a torrent of water, slowing down their advance.
"Laukaz!" she shouted, snapping her fingers to make an 'L' shape.
Oleandra marshalled the Thief's Downfall into snaking up the Trolls' large bodies to form great bubbles of water around their heads, preventing them from breathing. Instinctively, the Trolls began gulping them down, as one does, and the bubbles started to shrink rapidly, faster than Oleandra could maintain them with the number of heads she needed to cover and the amount of water the cup could produce.
"Isaz," Oleandra said coldly, drawing in her thumb to leave only her forefinger extended in the shape of an 'I'.
Unlike the vast majority of things in this world, water expands as it cools into ice. Bloody icicles burst from the Trolls' heads and throats, and one by one, they toppled, lifeless. Faced with this gruesome spectacle, the Muggles and Muggle-Born at the edge of the crowd shuddered, looking at Oleandra in fear, even though she had saved them all.
"Mistress Greengrass," said Julianne, her voice trembling as she turned to face her. "Please, the young lady, she went to face Voldemort on her own…"
She pointed to the north.
"I'll do what I can," Oleandra replied. "Keep yourself safe."
Oleandra flew off in the direction Julianne pointed.
As she approached the roaring wildfires, the temperature rose to almost unbearable levels, and the smoke began to burn her lungs, so Suit the Lethifold extended over her head and face, and she wreathed herself in water from the cup, protecting her skin and filtering out the worst of the airborne ash.
"Avada Kedavra!"
"Avada Kedavra!"
Below, a revived Voldemort and Mai were duelling, hurling Killing Curses at each other with murderous intent. The Heliopaths and Death Eaters had given them a wide berth; green beams flew everywhere indiscriminately…
…but despite seeming evenly matched, Mai was on the back foot. She was only thirteen; her magic was still immature. Before long, Voldemort would have her exactly where he wanted her…