Stand behind any mounted minigun on any level, and melee it. You can do this by yourself or get other people to help. Every time your gun hits the minigun, its launching strength will increase. Once you hit it enough, jump on top of it. This will either make you bounce very high or launch you across the map to your death. Bouncing straight up on it allows you to get to unreachable places such as buildings, airplanes, barriers. etc.
