Developers: learn how to properly manage memory. Flash doesn't do garbage collection until it's literally out of memory when trying to instantiate a new object, so you need to reuse your objects instead of using 'new' every time, because every object you make will be eating up memory and CPU cycles for pretty much the entire time the game is running.
So remember kids, if a man is willing to pull the very stars out of the sky for you, it's generally a bad idea to go throwing dynamite at him. Thank you Nerdook, you are a shining beacon of morality to us all!
There's way too much luck involved here. When everything starts exploding all at once (which is impossible to prevent on the later levels), it's far too easy for a single enemy to be launched straight past you with virtually no time to stop it. So levels that give you no spare lives can end at pretty much any time, just from a single unfortunately-placed blast.
I see a game like this, loaded with constant in-your-face "give me money" signs, and I can't help but compare it to the literally thousands of other games, on this site alone, that are just as fun if not more-so, that are also 100% free. What incentive would anyone have to pay for 20% of one fairly short game, when there's an absolute glut of great free games to spend time on instead? And, as others have mentioned, why should I pay for 20% of an amateur flash game when I can buy a full professionally-developed game for the same price? Honestly, when you put yourself into a such an over-saturated market, you need to be aware that overcharging for your content is going to make a significant portion of your potential customers view you as greedy and unlikable, even with the "fund my future projects" idea behind it.
Word of advice: when you've finished the level, don't go through the door immediately - blow through all your remaining AP first. With the AP restoring ring, I found that I can build up an Annihilate for every single level, making the endless dungeon trivial.
@Eclipse Angel, the controls are most certainly the biggest source of difficulty. Yes, the robots do exactly what you tell them, but 1) the commands you can give the robots are extremely limited, and 2) the mechanism for giving these commands (i.e. dragging the mouse) is painfully inefficient. Figuring out the strategy for a given level or challenge is trivial compared to wrestling with the controls in order to actually enact that strategy.