Well, I still don't know if anyone has solved this so I'm going to reveal the glitch used to start the level. If anyone has used this glitch before, please tell me!
GLITCH
The instant a fireball is reflected by a reflector, push the reflector one square to the right. The fireball will bounce a second time and blow up a box.
Yes, I knew about the glitch and used it from the beginning, it was just making it all together that took the time. Once on the actual mount, however, I made it in the first attempt.
Well done for completing it! The third mile is veeeeeeery hard...
It's one of the only levels that I've been really happy with the result, but JEU3 is looking good too!