You must be in the Renaissance path and have the University and Rocketry upgrades. The Logistics upgrade in the warehouse is also good. Enable the makemenails code (and also the gimmegimme code if you are low on resources). Produce three to five Sappers. Build as many Rocket Launchers as you want and one Supply Wagon for every three launchers. Make the wagons follow different launchers and send them to attack your enemy's manor. They will be invincible and kill all units around the target.