You might take a look at Pathfinder's Armies and Mass Combat
rules for ideas or inspirations. They allow you to treat each player as a Fine Army (1 individual), so it might be workable. I've eyed using these rules myself, and have contemplated sitting down, building a basic 'army'
and figuring out how a PC or three would stack as an 'army', and throw the former against the latter.
A few rolls would, I think, have players slashing through large numbers of the opposition while taking some
damage. Tactics - falling back and regrouping, healing up, etc. - may come into play ...
A caveat/warning. Just did this with two high-level characters (one Monk 20, one Rogue (Scout-Sniper) 15, thus Fine armies), each individually compared against a Medium army (#100) of baseline human mooks (Ftr1). Due to high movement, healing ability, magic armor and weapons, etc. etc., each high-end individual easily wipes the floor with the Medium army.
The mook army needs a natural 20 to even have a chance to hit either PC, doing its ACR (0.5) in hp damage, rounded to 1. In contrast, the PC hits even if they roll a 1, and does as hp damage the difference between their attack roll (1d20 + Offense Modifier (OM) ) and the opponent's Defense Value (DV, 10 + ACR + defense/settlement bonuses). As the mook army only has 5 functional hp ... immediate wipeout in the case of the Monk, 2-phase wipeout in the case of the Scout-Sniper, who gets a ranged attack anyhow
and so would do half the damage during the closing (ranged) phase, then the other half during the melee phase.
At a guess, presuming your PCs have some healing potions, magic armor, and magic weaponry, a character around 8th level (+/- 2) is going to have a challenge taking apart a 100-person army of first-level characters - might take some damage, but won't necessarily auto-waste everyone. +2 pc levels per army upgrade in quantity ...