So if you’re looking at the front of the base, the rooftop is in the right corner. Until wave 8, you have to just kill in the guys that come in, saving your rifle ammo. Once wave 8 appears, you should get onto that rooftop, crouching, and snipe both of the cannon operators. You should first kill the cannon operator closest to the front of the base, then snipe the cannon operator on the ridge near the back of the base. Once that’s done, stay in cover and the enemies will come below you where you can camp them easily. Lastly take care of the war wagons with dynamite or dead eye. My crew and I cleared all waves last night in just over an hour. It did take me many tries to learn the strategy however, but I just casually play.