Baba Yaga is an optional boss that spawns on the top of each hour in the Birch Grove.
To get there enter the portal next to the Birch Tree at Bored Beans.
Dr Defecaus is an optional boss located in the third area of the sewers.
You will need to kill him to get the Job Application for TP Pete.
Also known as the The Wolf Golem, Amarok is the World Boss of Blunder Hills.
Kill this boss to get the Desert Topaz for Builder Bird in order to unlock the portal to Yum-Yum Desert.
This big, bad evil undead looking elephant is Efaunt, the World Boss of Yum-Yum Desert. Kill this boss to get the Glacier Quartz for Constructor Crow in order to unlock the portal to Frostbite Tundra.