Maybe a new type of siege machine ? A pack-unpack catapult or balista? but a stronger one .
Something like in Age of Empires 2, the trebuchet you could make at the castle?
hmm isengard shoulda use cannons firing mines filled with gunpowder-like substance created by saruman... yeah imagine a mine-launcher cannon.. in any rate a strong pack-unpack catapult/trebuchet would be nice... i'm wondering.. wouldn't it work to tie a big rubber band from one ent's arm to another and treebeard to use it like a slingshot
Dude, this is the elven showcase, not isengard's
A onager could also do the job Ofcourse manned by Dunedain