TY the Tasmanian Tiger

Much like the other zones, complete the 9 missions and then beat the missions at the end of the zone. There is one timed mission at the end which is very easy to complete within the time constraints (no collectibles, 60 second time limit). After that, you face the boss, Cass, in 3 checkpoints. During the first battle, utilize the new boomerang given to you and continue jumping around the revolving landscape until Cass's shield is down, then fire at him. Rinse and repeat three times for the first part of this battle.

After you finish the first part you will reach a checkpoint where you now have to continue going through the revolving landscape and destroy 3 magnets by hitting them with your boomerang. These magnets are on the edge of the revolving landscape, so be mindful of the fire shooters, and take your time to destroy 3 of them. Once you do this, you will reach the last checkpoint, and 3rd part of the battle.

For the third section of Cass, he will now have the same shield from part 1,and now also have a flame thrower on his ship. Avoid him until his shield is down then lay the boomerang into him. Do this 3 times, and he will be defeated once and for all, giving you this achievement for beating the game. If you collected all the collectibles, and completed all of the bonus stages, you will have also gotten the "No Stone Unturned" achievement.