I went in there with 4 armies, Theoden with Rohirrim, and two mixed half stacks under Gandalf and Aragorn. Also had an Allied Army under Denethor sally forth. The enemy consisted of the Witch King and Gothmog, both with full stacks, and Amr, a Haradrim General with a few units and 2 units of Mumakil. It was about 2500 vs nearly 10 000.
I started with Aragorn (about 800 men) fighting the Witch King (about 4000 men). I was able to hold the line and soon reinforcements consisting of Gandalf, Theoden and a few units of Rohirrim, I was able to hit them on the flank and rear. Gandalf killed the Witch King, and his army broke.
However, Denethor had run into the Haradrim Army and the second Mordor army. I tried to take the pressure of Denethor as more and more of my reinforcements arrived, but I was fighting uphill and the hordes of Orcs from the Witch King and Gothmog armies held me back. Soon the battle turned into total disarray, and as Denethor's Army continued to diminish, more and more forces were sent against me and I began to become driven back.
Then the Mumakil came down and I decided to use my Cavalry to delay them long enough for my Riddermark Skirmishers to defeat them. Some of my infantry units began to route and I was starting to rely more and more on Cavalry. About this time, Theoden was slain by Trolls.
2 minutes later, Denethor is finally killed and his men flee, leaving me to face nearly 6000 Orcs, 10 Mumakil and a few hundred Haradrim.
What I didn't notice was that some of the Mumakil had circled and attacked Aragorn himself. If Aragorn died again, I'd lose the campaign.
So I quickly diverted nearly all my unengaged cavalry to save him, and soon only Aragorn stood left fighting the Mumakil. My Riddermark Skirmishers soon riddled the Mumakil with javelins and more reinforcements arrived, consisting of Dismounted Eorling Lancers, Eorling Swordsmen and a unit of Royal Guards. So I moved those forces to hold the Orcs at bay while I rallied my other cavalry for a mass cavalry charge.
Then, in less than 30 seconds, my reinforcements break and flee, with my already battered men. Only at this time did I realise Gandalf had been killed and I was on my own, with less than 500 men to fight the enemy. I decided to retreat as there was little else I could do.
I ordered Aragorn and the remnants of my infantry to retreat, while I let the cavalry stay back and try to kill as many of the enemy as possible before fleeing.
I had a bit of luck with that, killing the other Mumakils and destroying a unit of Serpent Guards, killing Amr in the process. After a while, I was down to about 50 Cavalry. Gothmog personally pursued my men as I continued hit and run tactics, and I was able to kill him.
Then, seeing the mass of Orcs approaching, I ordered my men to retreat, losing the battle.
Results: Nearly all my men slain, Theoden, Denethor and Gandalf slain. About 6000 Orcs and 500 Haradrim slain, Amr, Witch King and Gothmog slain.
Overall, about 500 of my men remaining against about 4000 of them. If I'd had sufficient forces I would've counter-attacked as the enemy had lost their Trolls and Mumakil, and their Generals for that matter.
But, I couldn't muster the forces and the campaign was lost