There wasn't really a challenge. I got up to about 327 before stopping.
Ironically, the worst part of the game seems to be the gravity feature. It appears the game's balanced around using it, but there is no reason or incentive to. It's an absolute negative. It serves to disorient the player, give the enemy more horizontal distance before they reach you/the ground, and cause you to miss more due to the small enemy hitbox. There are no bonus points for getting trick-shots or mechanical advantages gained by inverting gravity.
Enemies were predictable. There was no variety in enemies or movement patterns. They never really 'fell' farther than half-way across the screen, so you generally only had to nudge yourself a little to the left and right if you stayed in the center. The difficulty didn't increase over time either, making this simply a game of patience and stamina.