Really Big Sky is an indie shmup with a twist: it's procedurally-generated. Meaning you'll never play the same game twice.
The game's difficulty, levels and enemies are generated on the fly based on how you're playing, how well you're playing and how many of you are playing it at once (it support four-player multiplayer).
So you'll never, ever be able to "master" it by learning where every bad guy is going to come from and what form the bullet hell will assume. They'll be different every time! So it's just you and your reflexes.
You can find the game for $US10 from its official site.