Parallel Pixel, an infinite side scroller that ramps up the difficulty the further you get (like most games in the genre).
Destroy every enemy in your way to advance to the next area, and make sure to end Steven the Slime's gloopy existence.
Break apart police turrets that want to stop you from advancing, as much as a lifeless machine can 'want' anyway.

Destroy everything in your path to advance.

Now go forth, fight infinitely.