The earliest you can unlock this achievement is when you reach level 18 for your marine. Upgrading the combat pistol is probably is the easiest weapon to get this with. It has 3 upgrades needed for the achievement (skins don't count) and all of them are unlocked before any other weapon. From your menu, press on "versus" then, under the word "customization", highlight and press on "edit loadouts", "marine loadouts". Press to customize when you've highlighted the weapon bringing up your upgrade screen. You'll be able to select upgrades you've unlocked so far and spend points (located in your upper right hand screen) to purchase them. You need to purchase the attachment, sight, and fire mode for the combat pistol to unlock this.