On top of that you'll get about 900-1000exp. The conditions for this gun are explosion kill, more than 30m kill and kill from above, so each of this conditions are met with every explosion.
Now load and repeat the process, after couple to loads you will have all upgrades unlocked. Now aim for the gas canister near the truck that would trigger a massive explosion destroying the vehicle and standing soldiers. Go to the destroyed wall in the tower and save, if you were quick enough there should be 3-4 soldiers standing near the truck. In the first map after you kill first two soldiers you will find a sniping tower which you should run to get on top as fast as you can, the reason for this is to kill the soldiers who at this point should get off the nearby truck.
I would suggest using the first handgun you start with, namely Welrod Mark I.
I would advise to start doing this at the very first stage, maxing out the three main upgrades will take you about 30 minutes if you know how. This is one time consuming achievement that you may or may not unlock by playing because of the conditions you must met to upgrade you gun to the fullest.
