My strategy is to become a snow ninja so I can help other people with their health bars. Also I save up my power cards for an awsome combo! Also I would never fight alone if both of my team-mates are down then I would pump them up again :3. And obviously you have to co-operate and always spread out or stick together either way you, hopefully, would win and complete the bonus round! With this strategy I've completed it fully about 2 or 3 times.