I'm not sure slingers can make up for archers. They don't have flaming ammo and thus can't scare of elephants as effectively. They also have a very short range in vanilla R:TW. I'd personally, if faced with an enemy with slingers only, quickly destroy their missile troops and start raining death on their remaining troops, forcing them to attack or stand still and take plenty of casualties before I move in - or even worse - before THEY move in. In many 2vs2 battles in mp I've also been able to destroy an enemy, and be destroyed by an enemy, thanks to surviving archers when the rest of the army was destroyed. In mp, battles usually take much longer than in sp, so you often get to use up all the archer ammo.