Well. It's been said in other places, but playing Isengard can be rough...especially since you're depending on snaga archers and skirmishers instead of Dunlendings. It takes at least 30+ turns for the main Dunlending town to even increase in tier and give you more than foresters.
What I've had great success doing on VH/VH, is actually manage to ALLY with Rohan. It takes some work and quite a bit of cash, and a little bit of luck. But it's totally doable. I even got my Reputation to reliable from occupying towns and releasing prisoners.
What helped me do this was dashing up and grabbing Fangorn, and offering it as part of my alliance offer.
Meanwhile, you have to move fast to get Tharbad and Ost-in-Esthil before Eriador and the misty mountain orcs. The latter will always get to Ost first, but tend to be defeated, leaving it weaker.
After that, it is a mad dash to the coast...There are between 2-4 provinces to pick up there, down to the sea, and how many you get depends on luck, with the High Elves, Rohan and Eriador closing in around that time too.
Eventually, Rohan and Gondor will become distracted with Mordor and Harad.
Eriador will probably attack you first. Fortunately, their armies suck - even if they spam a whole lot of them. It can be tedious at the start, since Isengard is far from the main action, and it takes like 4 turns even to get to Staddle from your borders. You'll be fighting with A LOT of snagas, goblins and the occasional dunlending band.
But you should be able to eventually eat into Eriador, even if they constantly spawn 21-unit stacks.
The Orcs of the Misty Mountain won't do much except get the

kicked out of them by everyone. Even the dwarves alone will maul them. But they will still distract other factions for a while.
Eventually, around the time you've fought up to the Shire, the High Elves and the Dwarves will get involved. And at it this same time, your supply line problems will just barely begin to ease...as places like Bree start being able to put out something besides goblins.
But make no mistake...Even up until turn 100, 95% of your armies will consist of goblins and snagas, even when recruiting all the wargs and foresters you can, simply because Isengard is so far from the actions. By the time your Dunlending towns manage to grow (they start off incredibly weak), you won't need them. You'll already be able to get more and better troops from places conquered in Eriador on on flank, and Isengard will be churning out better troops for any potential fight on the eastern flank. So there is never any reason to recruit the clansmen and levy archers. Which is sad, and an oversight in design, I think.
You can apparently recruit Uruk-Hai archers in Bree, but not Dunlendings.
Anyways. That's what makes the 'Eriador' first strategy so tedious for Isengard. Your long supply lines and having to fight with stacks of 20 snagas/goblins. It's different with Rohan and Gondor, since Isengard is right there.
By focusing on Eriador first, Rohan and Gondor together will probably hold their own. In fact, in my current game, Gondor is number one, and the first evil Invasion has been called on E. Osgiliath. I've been in all out war with the remnants of Eriador (finally reduced to one province in the farthest north), a powerful dwarf faction and the annoying high elves. The OoTMM are close to breaking, but I think they might hold if the dwarves get distracted enough by me (they just recently came in to help the HE, after the HE came in to help Eriador).
I'll admit. It's been very hard and very tedious. But when I eventually have everything west of the misty mountains, I'll be able to combine everything in Isengard and really unleash hell. Maybe even supplant Sauron!
So allying with Rohan works.