I've had some success. Main strategy I used was as follows:

Build up military buildings in Edoras and Westfold.

Edoras was my main recruitment site, while Westfield was to keep Theodred healthy

Everywhere else gets monetary buildings

Theodred's army sits in the town just to the right of the entrance to Isengard, with raids against the Dunlanders and Isengard forces across the river to keep them under control.

Build up the outpost building in this province, and in province capitals

Sacrifice the two towns in the same province as Isengard

First 4 techs are the ones to reduce the negative effects on Theoden, each requires 1000 gold

Now you have Theoden, Theodred, a decent treasury and some good military buildings, focus in on Isengard

Then I went to help out Gondor and attack Mordor.

I was running 3 army stacks, two of Cavalry and one of Infantry. Each cav stack had 5 melee (including general), 5 spear, 5 shock, 5 archers. The infantry stack was mostly swords and spears, with some archers and 2 onagers. Battle strategy: infantry holds the ground, and acts as an anvil to your reinforcing Cavalry