My players have managed to convince a group of Onderon beast riders to help raid an imperial prison, fighting against tie fighters, gun emplacements and the odd walker.

My players forces now consist of a stolen Imperial shuttle, an armed freighter and 37 beast riders and mounts. The beast riders are currently following the freighter, this will probably be the imperials main target.

I was wondering if anyone had any ideas on a good way to handle this situation without getting too bogged down in dice rolls?

I had an idea that I could upscale the rules for swarms, to help group the beast riders attacks against the imp vehicles?

I think you definitely do not want to over complicate the encounter with too many dice rolls that the players aren't participating in. The massed combat rules out of Onslaught at Arda I sound like they would be perfect for your encounter. Just estimate the size of the enemy force, have a lead NPC for the beast riders, and a lead NPC for the Imperial defenders and you can make one roll for the whole thing. Maybe make a table of interesting uses of Advantage/Triumphs/etc and narrate the result of the roll off that (ex. Triumph on the roll, the riders make a thunderous charge against an AT-ST, toppling it in one pass and taking it out of the fight).

I've made the mistake of trying to keep track of too many enemies in the past and it ends up making my player's minds wander.

Always keep the focus on them if you can.

I would definitely keep the beast riders in the background, narrating what they're doing but not rolling any dice. If you want your players' actions to have an impact on them, give them some targets on approach, like air defence blaster turrets. If the PCs manage to take those out all the beast riders make it safely to the prison and are able to pull more Imperials away from the PCs; if they fail to take out the turrets, many beast riders are shot down, fewer of them make it to the prison and there will be more Imperials for the players to fight.

The AoR GM screen has squad rules, where the squad mates act as ablative plot armour for the PCs. There are rules for different formations that do different things, like skirmish that makes auto fire and blast cost more Advantages to trigger, or squad specialist where you make a Leadership check in place of computers, medicine, etc.

Only the player rolls, so the squad doesn't make their own additional roll. its a pretty cool set of rules.

My plan will be to have the beast riders initially attacking the 3 ties, whilst the players ship take on 2 interceptors. I am thinking that although according to RAW the beast rider weapons are not strong enough to damage the TIES, having 37 of them attack en masse will even things up.

I will probably then suggest that the beast riders take on the ground forces to hamper reinforcements assisting with the forces inside the prison, as I am pretty sure that my players will end up finding a way in to the prison to resue the captured Wookiees. At some point I will also have to deal with the 300 odd Wookiee prisoners going on a rampage against the stormtroopers guarding the prison.

Hopefully this will all be quite epic.

I ended up using a combination of the Mass Combat rules from onslaught at arda combined with some narative to keep things moving, it worked pretty well. Thanks for all the helpful tips.