In The Two Towers, while the Ents were attacking Isengard, the survivors of Saruman's routed Orc army attempted to flee into the strange forest that suddenly appeared near the Helm's Deep battlefield. They were set upon and destroyed by Fangorn's angry trees. How did the forest destroy Saruman's Orcs?
The trees in question were Huorns, though exactly what Hourns are isn't conclusively proven - we were only given Merry's speculation. He suggested that they headed off to deal with the Orcs
'But, though I could not see what was happening in the dark, I believe that Huorns began to move south, as soon as the gates were shut again. Their business was with Orcs I think. They were far down the valley in the morning; or any rate there was a shadow there that one couldn't see through.
Merry described them in more detail:
'It was the Huorns, or so the Ents call them in "short language". Treebeard won't say much about them, but I think they are Ents that have become almost like trees, at least to look at. They stand here and there in the wood or under its eaves, silent, watching endlessly over the trees; but deep in the darkest dales there are hundreds and hundreds of them, I believe.
'There is a great power in them, and they seem able to wrap themselves in shadow: it is difficult to see them moving. But they do. They can move very quickly, if they are angry. You stand still looking at the weather, maybe, or listening to the rustling of the wind, and then suddenly you find that you are in the middle of a wood with great groping trees all around you. They still have voices, and can speak with the Ents – that is why they are called Huorns, Treebeard says – but they have become queer and wild. Dangerous.
Gandalf backs up this supposition, implying that it was indeed the Ents that sent them:
The land had changed. Where before the green dale had lain, its grassy slopes lapping the ever-mounting hills, there now a forest loomed. Great trees, bare and silent, stood, rank on rank, with tangled bough and hoary head; their twisted roots were buried in the long green grass. Darkness was under them. Between the Dike and the eaves of that nameless wood only two open furlongs lay.
Gandalf laughed long and merrily. 'The trees?' he said. 'Nay, I see the wood as plainly as do you. But that is no deed of mine. It is a thing beyond the counsel of the wise. Better than my design, and better even than my hope the event has proved.'
'And what may be the answer to your riddle?' said Théoden.
If you would learn that, you should come with me to Isengard ' answered Gandalf.
Since they were basically tree-ish Ents, they could have killed the Orcs any way the Ents did - with brute strength.