|This achievement will be unlocked as you progress through Story Mode. Once you complete the game by defeating the Intruder it should pop up.|
Around level 50, you'll finally confront The Intruder for the first time. To fight it, bite the end of its antenna and lead it into a mine. Then grab the Shrink bubbles (mushrooms) and repeat. Once you beat him, you'll continue to the next level. You'll fight The Intruder multiple times, and the only thing that changes is which character you're fighting it as. Level 60 will be the final time you fight it, and thus the end of the game.
Cannot be unlocked in Time Attack Mode or Story - Lite Mode.