The final boss fight comes after finishing the last three chapters, That Lost Island, Crabby Convoys, and Fair Dinkum Drinking. Each chapter comprises three levels. The final level is As TY Goes By, and it's split up into three parts.
The first part that gets its own title, Around the Back Way, is a time trial. You have 60 seconds to get through the course. There's about 5-10 seconds leeway for mistakes. Use the Plasmarang, as it can stun underwater crabs without a power hit.
The second part is the boss fight with Cass. The stage is constantly rotating, and you can either fall to the right, which I preferred, or climb to the left. Every two missile salvos, Cass drops his shield. Hit him with the Infinirang. Three hits moves on to the final phase.
In the same rotating stage, flamethrowers are now active everywhere. Keep to the edges to find the three Magneticons. It's easiest to wait until they're at the bottom so you can run in and bite; it's possible to get some by falling to them, but others are too well protected to make it work.
In the final phase, Cass comes back and chases you with a flamethrower. I tried to keep him on the left side of the stage so I could always drop further down to get away from him. Like before, after a set period of time, he'll drop his shields. Once you get the second hit on him, you may be able to get a third in immediately to end the fight.
The achievement pops after the final comic cutscene.