itch.io

it utilizes some acs magic but heres the simple run down 

Bullet puff height (z value) is given to the monster, and the acs script is activated.

The Acs script creates a "head" range for monster (between monsters height + z value and monsters height - monsters height / 4 + z value), basically it gets the top area of the monster. If the bullet height value given to the monster falls in that range, it registers a headshot. The headshot in this case causes enemies under 70 hp to die instantly, and enemies above that to take 15 extra damage

Hopefully that explains it, acs is handy but also a finnicky thing, and even i was shocked that it worked. Just pray any custom monster doesnt have thier head on the lower part of the hitbox :)