Thanks for the comment, happy to see people are still enjoying this long after I released it! :) The shooting enemies off screen thing is something I have to use even when I'm playing those last few levels. They might not even be physically possible without it. :| Overall I just wanted to say thank you to people like you who took the time to care about the game and point out some of its fairly obvious flaws that I ignored. It's been a great learning experience and everybody who gave feedback made making this game worth it for me.
You're welcome :) ; I decided to get back to playing it since it had been a while since I last played it, and I wanted to complete it once and for all :D
Yes, that 'shooting enemies off screen' thing made it possible to get through the game, especially from levels 11 to 15 :p
It's good to know that you've learnt from that experience, and if you are going to continue making games in the future, I'll be sure to keep an eye out for the release of your new games :)