Upgrade these building in the exact order I write them down.
Armory -> Landing crafts -> Riflemen -> Gunboat -> Medkit -> Shock -> Barrage -> Artillery. Everything else is irrelevant. Basically when attacking destroy rocket launchers first then mortars and if flamethrowers and machineguns are still alive just shock them and heal your riflemen. Rinse wash and repeat. Have yourself a good day attacking.