The way I got mine was with the shotgun. Its extremely powerful due to the fact that you can auto fire it after rolling. So set up a shotgun match on pirate fortress or training facility, When you come across someone just immediately start rolling (Your invincible until the roll animation ends) and as you roll hold down the trigger and aim. It takes a little bit of practice but if done right Ive gone to about a 32 kill streak with it. Practice practice and it will be your new favorite weapon.