The Alchemeister is the Mini-Boss of the Explodatorium. Initially appearing as an old man, once he consumes a potion he transforms into a large beast. He possesses 23 hit points; more than even full bosses.
- In his normal form, he throws variously colored vials that explode when they hit the ground and damage the player if they hit him.
- In Specter of Torment, since the ground is covered with water flow, some of the vials may cause continuous explosion.
- In his beast form, he runs back and forth across the screen, increasing speed as he takes damage.