Is the Dung Defender optional?

Is the Dung Defender optional? Out of the above listed bosses, Mantis Lords and Soul Master are completely optional, Dung Defender is an either/or boss, and Broken Vessel is a “technically required” boss.

Out of the above listed bosses, Mantis Lords and Soul Master are completely optional, Dung Defender is an either/or boss, and Broken Vessel is a “technically required” boss.