During the last level against Tower Tooth, all you have to do to defeat him is drive around and pick up power missiles until you reach the hold limit. Then, drive up one of the stair ramps that takes you to the raised platform around the buildings and park your car facing the ledge. When Tower Tooth arrives, he will sit directly in front of the ledge and attack you. Face directly at him and keep shooting him with power missiles repeatedly. When your health gets low, set down a remote bomb and detonate it before Tower Tooth has a chance to get away but not so soon that you are also next to it. Drive around until you have full health again. The white "+" on your mini map represents a full health pickup. Gather more power missiles if needed and repeat the process as many times as desired. If you are away from Tower Tooth too long he will regain health.