Wow. Replicating the Battle of the Pelennor Fields... I don't envy you.
Well, in all honesty, they'd have never won that one if not for Pippin, Merry, Eowyn and the army of the dead. They were fighting against admittedly impossible odds, and Minas Tirith's structure, while in the strict defensive sense is very formidable(that is, if they lost too many troops to hold the largest level, they can fall back to the next one, prolonging the inevitable if nothing else) losing a level has negative inpact in both morale and resources, and once the first one is lost, they will be cut off from the outside world, save for some hidden paths in the mountains.
Aside from assassinating or bringing down the Witch King(Yeah.. good luck with that), you'll have to go for a number of small victories and backup plans the original party didn't think of or never bothered to try.
1. First and foremost,
you'll need forces. Lots of them. Get as many a soldier to your side as you can. Dwarves, Elves, Ents, Halfings, Mountainers, Undead, Trolls, Haradrim, Dunlanders, even Orcs and Uruk-hai, anything you can pull off. Ensuring the Rohirrim arrive before the first level falls(if at all, but don't expect miracles) might be a nice boon as well.
2. Save those who fell in the original trilogy! For example, help the defenders of Osgiliath pull back! Next to all of them are hardened rangers, fighters and warriors, I'd say at least 4th level each; behind walls, those guys are killing machines compared to a bunch of 1st level suckers whose morale is down the drain. And at that...
3. Demoralize! The orcs are barely held in line(if you can refer to what they have as a "line"), hate their leader, pretty dumb, and generally would have preferred to stay at home with their families. Thus the most effective tactic against them is to piss them the hell off. It doesn't matter if they get angry at you or Sauron, you can easily abuse both. Eventually a fair portion of them will just desert or start to act on their own accord. Careful, they might have Barbarians.
4. Cut the supply lines from Mordor! You may have noticed the opposing army is huge - which means there is an even larger workforce to keep it in one piece, carry ammunition, housing, backup forces and weapons, possibly along with emergency rations in case they ran out of corpses to eat, and that's only the orcs; the Haradrim are obviously kept in better shape. I don't expect either to be any good at logistics, you'll have an easy time.
5. Don't let Denethor run that stupid mouth of his! It might be a good idea to steal his Palantir before Sauron sends him the magical equalient of propaganda; he might even be useful to a degree(again, don't expect miracles). If you can't, just knock him out, Gandalf-style, as soon as you can.
6. Protect the Gondorian trebuchets! Those were a powerful asset until the Nazgul destroyed them.
7. Tinker up new ways of mass destruction! Knowledge and Craft checks are your best friends. Why rely on magic when the real world is already ridiculous?
8. "Kill'em! Bone them! Cut them to bits!"

Or let them surrender, whichever you preferred at that point.