You'll need to complete 2 levels: Mission 11 - Storm the Palace & Mission 12 - Zartog's Demise.
Mission 11 is similar to the other levels you have played thus far. Work your way though the map collecting the banana and globhoppers along the way. Nothing is too challenging here that will prevent you from completing the mission. It will be one of the harder "racer" missions when you try to go for the 5 minutes or less achievement.
Mission 12 is your first real "boss fight". You'll square up against Zartog on a ....well, square. Run away from him in a circular motion across the outside to avoid his attacks. When he shoots green bombs, pick them up and toss them at him. Rinse and repeat until he is finished. After you do enough damage, some balloons will start to spawn. AVOID THEM as they do damage when you pop them. Again, the level isn't too hard and should only take a few minutes.