Ok so I was directed here as I am wanting to share something I discovered today. I did a bit of research before posting, and I can't find this documented anywhere so I figured I would submit it here for others to check out. Watching Any% speedruns out there for this, I grew even more intrigued as nobody was using this glitch. So I figured that either a) nobody knows about it, or b) its not useful. So if its already well known and I am just an idiot as it's not useful, I will happily delete this and move on :) It should be known that I am not a speedrunner obviously :)
I made a crappy video of the glitch here.
https://www.youtube.com/watch?v=Tgn-Ktl7Oyw&feature=youtu.be
At the time of the recording, this was the third repeat. I did nothing special to set this up, just picked Flame Mammoth's stage, fought through to this point, jumped on the smasher and shot it. Next thing you know I was given the stage clear screen and the weapon. I then went back inside and it repeated no problem.
I haven't tested to see what other weapon combos this works with or if you play the stage later