Alright, some generic stuff on using Isengard, followed by specific instances of use.

First, use the Phalanx against three things- things that you will grind into the ground (Warriors of MT)- basically anything that you have a higher fight value against, or more attacks, and use it against cavalry. Do not use it against large models- it is a waste of bodies and a very bad formation for taking down the enemy monster.

Personally, I always use Saruman- use like any magic user. As for heroics, I would also use Mahur- the one uruk other than Shagrat with three attacks. If you do go with Mahur, upgrade all your scouts- they defend the flanks of a phalanx and make excellent skirmishers.

If you can, use crossbowmen as your primary (or only) archers. Berzerkers are, in my opinion, overrated, but they can be highly intimidating against anyone who hasn't fought them before. They are great in WoTR, but not skirmish. However, feral uruk-hai are very nice.

As for general tactics, use your troops supported by Saruman to smash the enemy troops- the idea is to acquire a numerical advantage. Use D7 captains to hold off their heavy-hitting heroes until you acquire that numerical advantage. Then swamp them with models while they are immobilized. That is the use of Saruman and the horde, and how you take down Treebeard.

I personally do not use the troll, but that is a personal preference. If you use the troll, it is not quite like in WoTR- use the troll to take down expensive models (back him up with Saruman) or use him for captain hunting. Or have him be the hammer that you use to hit the enemy against your phalanx of an anvil.

Yep, Ferals have no weapon options...with 2H they'd basically be Berzerkers