This is probably a world record or something (see hi-score in the screenshot below).
Anyway, I disagree with the previous comment: it is definitely worth killing enemies, as they remain between stages. If you can keep the enemy count low, you can continue indefinitely. It's possible that the spawn frequency increases as the game goes on, but if so, it's a very slow increase, and I got bored before I got killed. Enemies can spawn very close to the player with no prior warning, which is what eventually killed me.
The safest (but slowest) method to increase score might just be to stand in the middle, where no enemies spawn, and just kill the enemies that spawn indefinitely. Only the player's tolerance for milking would limit the score obtainable in this manner. This is assuming the frequency of spawns don't increase (at some point, they'd spawn faster than you can kill).
