Had this trick with a friend a few years ago. Level 10 is the only boss stage where you can jump off and die, meaning you can get faster times. My PB (WR on Cyberscore) is 11.83s with this trick.
Just got back into it, so wicked rusty (and the fact that all my best times are on my 2DS).
Yeah. The timer pauses when you die. In order to get the time to count, the animation for completing the level should start.
I found a glitch that someone has covered on youtube, but ima see if I can post my own thread on it soon.
I'll see if i can get a video of it. You die after the boss has been taken out. It stops the timer and you can get low times like the times in my pics.
https://imgur.com/a/di2inwY Have a pic so far. I have the upgraded weapon after I beat the boss and the Stage Clear transition started.
Thanks. Me and pikacshu found this trick at about the same time and dueled for about 2 weeks or so.