At the Loading Dock, you cannot hope to take this monster out with any weapon. Your best bet is to get the minigun before entering the Loading Dock and shooting the explosive canisters that are circulating over the guard. Make sure to stay out of the line of fire. Keep hitting the canisters until the guard falls. At the Fuel Station, circle around Heavy Riot Guard counter-clockwise and blast the fuel tanks on the rail above it. You do not need a direct hit to blow it up, but it will speed up the process.