I found that the superior tactic is to stand in between the two gray cubes and shoot without moving. Getting hit is no big deal because health pour down on you as you destroy balls. also the balls seems to spawn always in front of that opening between this two grey cubes which makes the strategy even more superior.
As other people, it feels like waiting for the blue ball to spawn which is out of the control of the player and not much fun.
I'd say, if you can find a way to give the player insentive to move around move and give him more interesting intermediate goal, I think it would be more interesting.