It depends on the map really. The hardest one (next to the palace) was the Road to the Air Port. For that one, I only had six mercs. I put three on one end of the map, and three on the other end of the map. The first three had my main, Arny, with his shot gun, and Jabby. All the soldiers started going towards the northwestern part of the map, where my main was situated. So, I had Moby Dick, Rhino, and Castor, with his grenade launcher, and they killed or wounded a lot of the soliders. But my main, Arny and Jabby hadsix guys to deal with, four of which ended up on top of my guys, before we even saw them, so I had jabby stab one, arny threw some grenades, and my main, used his sniper rifle to tear them up. I had to reload a couple time though (one time a grenade hit a tree and went back on my guys lol). It was exciting though.
The airport was the easiest, I essntially had a killing field as the npcs ran across the air field trying to attack.