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 :)