Its very easy to cheese this trophy just like most challenges in the game. Once you equip all parts on to a weapon it checks it off in the challenge, even if you restart the game and don't buy the parts again. So the easiest way to get the trophy is the beat the game once like normal and then use challenge points to buy the WCX. When in game buy a weapon and all its parts then restart, selling all other weapons if you are strapped for cash. Doing this for all weapons and the S.T.A.K.E. will get you the trophy. While you are at it you should fully upgrade the guns as well which will unlock the ability to buy infinite ammo for the gun, even after you restart and no longer have the parts or upgrades.