If you have a pair of Xbox Live accounts, you can try this technique. If one account is of a lower rank than the other, you can give the lower account an instant power-up when using both accounts to play. When you unlock something at the end of the game (such as the Overkill Perk), immediately log out of your account and sign into the lesser account. Head to the Xbox Live game lobby and press Y, and the game will have unlocked for this particular account whatever you unlocked with the first account.