I keep exploring Feaster's Lair now that air dashing is an option and got to a place where you can fight another boss called: Misbegotten Amalgam. It is quite hard, as he won't stop attacking and it may take a while to learn its attack patterns, but it is doable nonetheless.
I didn't know this was an optional boss and, more importantly, a MISSABLE one, too. Supposedly, it disappears later in the game, a bit after leaving the Carven Palace, so if you haven't beaten it before that point, you should go back and do it then.
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