When fighting Wracktail, use the same tactic as on Fracktail, the first boss you fight. The best way to kill him is to get on to his back like always, change to Luigi, and go underneath the yellow ball on the antennae. Use Luigi's super jump to hit the ball from below but the you will stay in the air and start bouncing on the ball, causing serious damage to Wracktail. Using this technique, Wracktail should be finished within three times of doing this. He was dead in three times for me, but my attack power is 13 for Luigi, Mario and Peach and 26 for Bowser so it may take longer for you to kill him if you are not in the 20s levels.