I'll just post my suggestion here: Use ranged Heavy Infantry, or Artillery. These guys are basically only good for siege, cavalry, and Light infantry. 30 damage isn't really that much unless they're popping out hits every half-second.
Also, try to remember to build emergency barracks towards the back to allow yourself a means of building up an army, even when you're under attack. Be sure not to throw a small force in against a much larger force, even if it seems like your units have a type advantage. Just let the deathball build up in size, then attack.
"You're assuming three people who barely have the resources to manage three boxes will get a telescope and watch from atop a building"