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. 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. I would suggest using the first handgun you start with, namely Welrod Mark I. 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. 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. Now aim for the gas canister near the truck that would trigger a massive explosion destroying the vehicle and standing soldiers. Now load and repeat the process, after couple to loads you will have all upgrades unlocked. 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. On top of that you'll get about 900-1000exp. This will unlock the master skin requirement and this is 100 head shots. This perhaps you could stack in the rest of the game as the gun is pretty darn handy due to the suppressor so you will eventually get the require head shots. If you don't upgrade all free weapons stats this will not count so keep that in mind.