I edited the Big Boo from icegoom's Super Mario World Redrawn project to work with ywkls' Acheman script. I'm super glad that these 4x3 Big Boo sprites existed because they are the perfect fit for something that can become invulnerable. The boss uses the first three sprites facing left and right, but I included two alternate "hiding" tiles if you want to replace the third frame with either one of those. I also edited an appropriately will o' wispy-looking fire from Super Mario Land 2: Six Golden Coins for the projectile.

Of note is that the sprite the boss uses for the projectile is taken from weapon sprite 40 (Din's Fire Trail: Rising), so be sure to edit that part of the script and change the number 40 to the number of the weapon you want it to use (assign tile and animation data to one of the z### weapons, then copy the number that that weapon is to the Acheman script and replace the 40 with that number).

Icegoom (original sprites)
Nintendo (Big Boo character and SML2 ghostly fire)