The easiest and earliest one to get this is for the Weapons MODs, as you simply need to get 5 gears on the first 4 Shooting Galleries (handgun, shotgun, machine gun and revolver). You unlock the 4th one after you finish the first mission of Act 3 (where you have to escort a plane). Alternatively, if you're having trouble getting 5 gears for each challenge, there is a 5th shooting gallery (burst), but it shouldn't be too hard to get 5 on the first 4 with a little practice. Once you reach 20 total gears, you unlock the trophy, whether you actually activate the MODs or not.