I have an encounter planned tonight where there will be factions of NPCs (groups of 20+) fighting, with the PCs caught in the middle and most likely supporting one of the sides. What is the best way to do this sort of combat, given that there are several rival/nemesis type NPCs on each side? I'd like to avoid doing 6+ dice rolls each round for the NPCs, is there some way I can reduce their combat down to some dice roll that determines something like... I don't know, how hard each side is 'pushing' against the other?

Personally I would just narrate most of it in between the PCs' actions.

I recently had the PCs stumble upon an Imperial raid on a Rebel cell, with 20 or so Imperials and 15 Rebels.

I had story objectives I wanted to do, and I wanted the PCs to play the major part in the encounter, so I just narrated each time it was the Rebels' or the Imperials' turns. Generally it just meant "one side loses a minion".

"The Imperials fire a volley at the Rebels, and one of the Rebels tries to duck out of cover to fire, taking a blaster bolt in the chest and collapsing."

I would group the NPCs into minion groups of 3-4 each, but only roll for those groups in combat with or directly adjacent to the PCs. for the other groups I would either hand waive their rolls to keep the battle balanced or to make the battle go the direction I wanted it to go.

If you don't want to dicate the outcome, I would preroll the battle without the PCs present and record the results. Then when you play with the PCs I would use those results to narrate what is going on around the PCs. Any actions they make will affect the battle so you will have to make adjustments on the fly, but it will give you a good starting point.

With the AoR GM Toolkit they added Squad/Squadron rules that work perfect for this. It turns large groups into 1 roll type situations with formations to handle the extra characters.

As to the "how hard each side is 'pushing' against the other" an opposed check could fit for that. Maybe a vigilance versus vigilance roll using the leaders of each group to oppose each other for the roll?

Mass combat rules are allegedly on the way, but one way you might determine the changing course of the battle is to come up with some easier roll. My recommendation would be an opposed Leadership vs. Discipline check between the commanders on both sides. Keep a Wound threshold handy for each that encompasses the entire group -- maybe with a value of 1 per Minion, 3 per Rival, and 5 per Nemesis. At the top of each round, have two of your players roll off to represent the changing conflict.

Successes cancel each other out, as do Advantages. Whoever wins does damage to the other side equal to their uncanceled Successes. Whichever side has the most Advantage can use their uncanceled results to gain ground, take objectives, or do other things that might add a Boost die to their side on the next round. Triumphs can be used by either side to accomplish things that could turn the tide of battle, and maybe upgrade their next check.

Letting the players roll the dice will help them feel like they're still involved, but unless they have a really good idea, you should be the only one dictating how the battle plays out.

Unless you already have an idea of how you want the battle to go, in which case, screw all this and just narrate it.

Successes cancel each other out, as do Advantages. Whoever wins does damage to the other side equal to their uncanceled Successes. Whichever side has the most Advantage can use their uncanceled results to gain ground, take objectives, or do other things that might add a Boost die to their side on the next round. Triumphs can be used by either side to accomplish things that could turn the tide of battle, and maybe upgrade their next check.

I like this idea. I do have an idea of how I want the battle to end, but I would like to have some 'randomness' in how it actually plays out. Mostly, I'd like the opportunity for the PCs to get hit really hard (potentially hard enough to generate critical injuries), but I don't want to crowbar that in at some arbitrary point, I'd like it to be surprising.

I had story objectives I wanted to do, and I wanted the PCs to play the major part in the encounter, so I just narrated each time it was the Rebels' or the Imperials' turns. Generally it just meant "one side loses a minion".

Something simple, like this. GM's should rarely, if ever, roll dice against themselves.

More importantly, what is the dramatic question being asked by this encounter? Is the rebel/imp battle scenery, or front-and-center? Is the dramatic question, "Will the PC's watch the Rebels get slaughtered, or will they help?" If so, then progression's idea works really well if you start with, say twice as many imps as rebels so that it's obvious what will happen if the PCs do nothing. Further, it's clear that either side has only mooks, so the players know that they can make a difference without it being an "unbalanced" encounter, especially if they can get the drop on the imps. (Of course, I would do something like...if the PCs blow away the imps, then a real fight stars against an obvious star player for team galactic empire while the rebels escape to continue their mission...)

However you do it, there should be a consquence to the outcome, making it a good branching point mid-way through an adventure and a great way to introduce the rebel alliance to the campaign.

My group is currently staging an assault on an Imperial compound. They will be accompanied by a platoon of Rebel soldiers made up of 5 squads of 5 soldiers. They will be facing a roughly equal amount of stormtroopers. Additionally, each side has a commander and some heavier weaponry.

I plan having each group of minions target another group of minions. The checks for each pair of contesting minion groups will be a Ranged (Heavy) vs. Ranged (Heavy) roll. The Rebels would use the positive dice (yellow, green, blue) and the Empire will use the negative dice (red, purple, black). The groups will deal outright kills instead of wounds, and the dice pools will be adjusted accordingly to however many minions are killed as a result. Commanders, the heavier soldiers, and the PCs will act in their own slots, targeting whatever units they want and following normal combat rules.

My goal is to reduce the amount of time it takes to run the large amount of minions, bringing it down to one roll for each pair of clashing minion groups. The large number of soldiers is mainly there just to illustrate the scale of the battle. The PCs actions will be the main focus, of course.