The approach I'm taking is a modified Victory Points system I created a couple of years back. I think it works well.
SPOILERS - LOOK AWAY IF YOU DON'T WANT TO SEE
The big battle features everyone still alive at the end of the saga. If they were an important part of the saga, you'll see them there. Disaffected generals, unfriendly elven rulers, Gate Pass resistance fighters, brothers of defeated archmages and their monk-warriors, Ragesian Empresses, Lyceum mages, allied nations and so on - much mroe than that.
Essentially, you need to gain a certain number of victory points to win the battle. Your victory points start depending on what allies you have made over the saga, and then change daily. If they reach zero, Leska wins and rules the world; if they reach a target number, Leska's forces are permanently defeated.
There are, essentially, three sides - the heroes, the elves and Leska.
Each day, victory points can go up or down, depending on:
- daily random events (Rantle has fallen in battle!)
- strategic missions (defeat enemy heroes and leaders, break or defend a supply line... or, favourite, convince Longinus to pilot what's left of the Tempest for you!)
- recruiting allies (rescue Shalosta from the enemy army and gain her force of treants and archers; convince a disaffected general to switch sides; what will the Shahalesti do - help you, or wait it out and take on the losers?)
It's a lot more involved than that, but that's the basic concept. I want the PCs to feel they are in the middle of an epic battle where anything can happen, and anyone can die - and that their actions are vital to the success of the allies.