How to run a big scene without bogging down?

[Ilium]

Pkitty is right. But also try to have different factions have different combat tactics. For example a wizard cabal will use minions or summons as fodder and will blast away from that defense perimeter. A lawful army will do formations like shieldwall with reach weapons behind it. Chaotic factions will have beserkers charging in. A rogue guild will have loose teams try to gang up on people. etc.etc. It makes for a much intertesting visual picture.

This sounds very much like a D&D Miniatures warband, and leads to another option. You could use DDM miniatures and rules (if you have them) for everybody but the PCs and the important bad guys (faction leaders and big critters). Let the players help run the DDM factions as suggested.