As the game tells you several times, this is the final quest- finishing it changes the world irrevocably, and blocks off your access to all other story quests. So if you have even a passing interest in completing the rest of the story (or the powerups gained thereby), do that first.
After a stirring speech (or as stirring as it gets for a vengeful undead warrior leading an army of monsters), charge Sauron's legions.
You can expect to see some Nemesi here. It may even to be advantageous to, during the course of the game, leave a few low level nemesis orcs untouched, so they can show up here as sword fodder. You can also recruit enemy captains as normal, even some familiar faces.
Defeat this first wave of enemies to face a second just like it (three captains leading grunts), and then defeat them to have Isildur take the field.
Isildur's tactics haven't changed much from before, but this time you have an army backing you up.
Counter and stab him as per usual, and run to Eltariel's shield when he starts in with the teleport bombs.
Repeat this cycle until he can be banished.
As with the others, you'll get a slice of the Big I's history (as well as how he came to be a Ringwraith- something Tolkien glossed over). After that, the narrative takes something of a turn.
When you regain player control, your mission is to recapture Minas Morgul. The basic mechanics are the same- cap each point to control the fortress, but this time instead of planting the Bright Lord's banner on each point, you'll be raising the dead.
It's not strictly necessary to banish all the nazgul you encounter, but it does tidy up the field somewhat.
When you cap the last point, make for the Upper City.
Here you fight the Witch King, a battle that is almost entirely about dodging.
Whenever you see that dodge prompt, dodge. At first it will be just to evade the blade of the Witch King himself, but as you continue to damage him, he'll keep summoning more Nazgul to hit you with ranged attacks, and you should dodge those too. When the King is not swinging wild, hit him with regular attacks, and always take the [E] counter when it comes up. Mechanically this fight is not that hard, but without regular orcs around, you can't easily refill health or ammo. So just take it slow, and dodge often to achieve success.
This ends Talion's part in the mission, but a sleight of far-seeing will allow us to finish up with Cel as well.
He may be haunting a different body now, but he controls exactly the same as he has up until this point, so no worries there. The important thing to remember about fighting Sauron is this- he's a big pushover, not even a vault breaker! The only thing that makes him dangerous is his lone special attack- rings of flame that race out from his person. These are quick, but it is possible to dash/roll over them.
Much easier, however, is to simply use Shadow Strike to close with him.
Save your elfshot for just such maneuvers. When you do get close, just smack him around with your sword (he's not a great fighter- rarely parries or dodges, and his own attacks are easily countered). When you've damaged him enough, he'll shift to his armored form.
This form is slightly tougher- keep on your toes for dodges and counters, because his blows pack a punch. You can still 'vault' him easily, but don't press your luck trying for long attack chains, just take your couple hits and then dodge. Also note that Eltariel continues to deal small-but-non-zero amounts of damage, and she can be used as a distraction to give you recovery time or backstabbing opportunities. When you've dealt enough damage, a short cutscene plays, and you face Sauron back in his gift giver form.
This time he has two rings of flame racing across the floor, but again you can use Shadow Strike to get in close, and apply the pummeling. Besting Sauron completes the mission and the campaign proper , leaving only the final act of the Shadow Wars.