Getting back into the game for fun. I remember Modular5 showing me how to skip level 2 boss a decade ago but he thought it was frame perfect if my memory is correct. Watching current runners doing it I'm not sure how to get it to work. I have been trying it with no luck.

You don't seem to be on the discord. I recommend joining it for questions like this. The link is below the name on the game, towards the top left of the screen near the cover art (I would post a link but I got banned when trying to do that before for something else so I have to explain where it is instead).

Anyway you're question. Yes the trick is frame perfect. You need to shoot the LETHAL homing launcher shot before the screen goes all black. The timing is random but Maurga uses an OBS setup to monitor the games audio to make it consistent. The link to his video showcasing it is below (it has no audio btw which is kinda ironic lol). As you can see he presses the shoot button when the grey bars vanish and before the screen goes black. Not much of a runner myself, but lemme know if you're still confused but I'd recommend asking on the discord to get faster responses.