Good concept need btter AI and variation
The two main problems is the AI and the repetiveness.
Useally the AI will rush towards the balls and all you have to do is hit him once.
But often you don't need that. If you just hide the AI will rush in between the balls and suicide while you defend yourself.
I think none of my games were decided by shield energy. the AI simply didn't live long enough.
The gameplay is totally identical. And what is worse so is the graphics. you have no sence of progress. After you get to five balls it fells like fighting the same battle over and over.
Some graphic changes would be a minimum. Gameplay progress you make it fun.
And if you are going to just go simple levels, add one more ball each time, sometime rearrange the pillars, then make it clear that you are fighting to level 20 or something. so people have a goal. There need some motivation to stay in the game.