Before you use the crank and send Rebecca down to retrieve the item make sure that Billy has the Pistol with 15 Rounds, grenade launcher, and shotgun. Depending on difficulty will determine how much you use.
Once you use the crank with Billy and Rebecca gets the item, Centurion will appear and grab Rebecca. Place all 15 rounds from the pistol into him and then switch over to the grenade launcher. If playing on easy two grenade rounds should do it, normal all six, and hard will take an additional 5 shotgun rounds.
There you have it! You have defeated the Centurion. Congratulations!