Depth. If I moved towards the right, enemies always came from the same directions (upper left, left, and lower left) which made the randomized aiming a non-issue.
Having found an optimal strategy that fast, is not good for enjoyment.
I feel that that is a good strategy,
But really the game is programmed so at any point 5 enemies could spawn in less than a second and you would be in trouble, but the chance of that happening isn't really high.
Maybe changing the equation to be in the area in which the player is moving to could help.