itch.io

You can beat 15 with 2 bullets by shooting the glass under 2 guards, (they take a tick of damage as they fall) and then throwing them in such a way that they glitch past the hitbox for the middle and corner glass panes, breaking the glass under the remaining two guards with their bodies.

There's also an even glitchier way to beat it with 1 bullet. I haven't figured out exactly how this works, but if you break the glass under just one guard and throw him at the glass under another guard, then grab that guard in midair, you sometimes enter a state where you've won and lost simultaneously. (the victory and loss text overlap at the top) At this point you can press the spacebar to advance. As far as I can tell, only 2-3 guards are dead in this state, but the game thinks you've killed them all for some reason.

Unfortunately I doubt there's a way to do it with zero bullets. The guards are all armored and I haven't found a way to glitch a kick through - mashing right-click and R at the same time doesn't work; it seems like the kick gets cancelled as soon as you enter the "seen" state.