I'll be honest... the last level is a PitA. Kaos's attacks can kill with one hit, meaning that you'll either need to restart the entire level, or have more figures. (It took me more figures than I care to count.) Be VERY careful when the various lasers/sharks/circles fly at you.
Edit: I've seen that the Kaos trap is available separately, so the Dark Edition isn't the only place to get it. I would absolutely recommend getting that trap.
For me (and you may not want to hear this), the key was to have the Kaos trap that originally came with the Dark Edition. Evolved Kaos is a beast, and did almost all of my damage in the final mission. You have to have the special Kaos trap. You can evolve him in the kitchen at the Academy. He works really well if you can press X and put an elemental circle right below Big Kaos, and then spam him with sharks. Over and over and over again.
Best of luck!