You mention using Isengard which is great as they are basically custom built to take down monsters.
Their S4 crossbows mean they can get cheap wounds on anything D6 or less from range. Vrasku can really help with this, don't be afraid to use his Might to inflict wounds on a big beasty from range. In one game (750 points) I took down Treebeard and an Ent before they got to my lines purely with crossbow fire.
In terms of combat follow Southy's advice, specifically to your army:
Use Sharky or (preferably) Saruman to Immobilise the beast then swarm it with a Captain, Ferals and Beserkers, they're Courgae 5 & 7 so should have no trouble charging the beast and that's all assuming you haven't got Chanelled Fury in play making them auto-pass. Once they're in, feint with the Captain (assuming the monster's F6 or less the Fight value of the normal uruks will beat it when immobilised), depending on the captain you'll have 2/3 S5 attacks re-rolling 1s with 2/3 Might to boost them. Feint with all but one of the Ferals and all of the Beserkers. Go 2-Handed with the Beserkers. You'll soon have a dead monster. If you can't immobilise him then call a Heroic Strike with the captain and follow the rest of the advice above (except don't feint with the captain obviously!)
Isengard - slaughtering monsters since 2002
