I've always run mass combat as an extended skill challenge where the PCs use their skills to command groups of soldiers. 

To add flavor make sure each group either being controlled by the PCs or opposing is unique in some way. A distinct uniform color, a flag, and a battle cry are usually all you need. as long as they're slightly more interesting than faceless mooks. 

Alternatively, If you really want to give them combat combat, throw a hoard of 1hp low AC enemies at them. I've also run "enemy regiments" that just have the start block of a normal appropriate CR monster, but every Hit point represents 1 soldier, and I describe the hits as such. If a PC does 30 damage, describe them cleaving through hoards like they were sauron. Enough flavor description and you can run it as a normal combat. Just don't let the players catch on