i mean, obviously going for MH2 zip would be a glitch, but is killing the boss too fast allowed in glitchless if you just don't do any shenanigans regarding messing with the level layout to do MH2 zip
You mean the way we do with Hyper Tails (the flickies going off screen) or with Metal / Hyper metal due to his double jump giving him an extended hit box? As far as I know, that's not a glitch - the bosses in all levels have their hitboxes active even when they're off screen. I'm not a mod so I don't decide whether it is or not a glitch. I just personally think it isn't.
Another example is Big Arms with Hyper Metal, where you can get a couple extra hits on him off screen.
A Glitch is an exploit of an unintended mechanic that causes the game to perform in a way that is outside of normal play.
As a result, i don't see why MH2 quick kill would be an issue, since the hitboxes are loaded, and since Hyper Tails can Hit him outside of the camera bounds, why it would be a glitch. Just seems more like an oversight, which is a different story.