I scrapped my previous plans for boss (sprite) design and went with something a little cleaner. The first boss's sprites are done-- there are fourteen sprites for this first of the four projected bosses.
Enemies and bosses alike in this game are squishy pink tentacle things with armor, and when you kill an enemy, the armor falls off into multiple pieces-- the bigger the enemy, the more pieces. With the bosses, the armor falls off during the course of the battle-- a chunk here, another couple when you reach this threshold, etc., giving the player a greater sense of accomplishment. At least in theory.
Will post more, including pictures, when I have more to post.