We wanted to do a boss (preferably big) from the start, and it was up to me to draw whatever it would be. My first idea was something like Dr. Robotnik's mechs or robot pods from Sonic, but then I stumbled on an awesome boss sprite from Sonic Advance 3 with floating hands and no arms.
It really reminded me of classic bosses like Mega Man X's Sigma & Mega Man 3's Gamma (basically giant robots that take up the whole screen & are composed of floating fists & a head weak point). I asked the other guys what they thought of it, because I knew it would take a while to make, and they both vetoed it. So obviously I made it anyway.
To speed things up I lifted the Sonic Advance sprite's general design. It wasn't the scale I wanted, though, so I used it as reference to redraw the hands and spine. The barrel body is custom but pretty much the same shape. Once I came to the head, I asked the others what it should be like. Seb noted that this is essentially the Earth's "last line of defense" so it should be something big and heroic "like a Gundam or something." With less than a day left it was both easier and funnier to just directly rip off the iconic Gundam head without even explaining it in game, so I went and found a Gundam model photo for reference and made a really obvious Gundam head.
I made it pink just because I thought it'd be amusing. The addition of the pink-haired samurai girl in the corner, shouting orders Giant Robo-style, is actually a last-minute addition by James, but that all ties together really well. Seb also did an awesome job animating the boss -- it was his idea to have the spine snake around, and to position the boss behind one row of buildings for effect.
One thing I really wanted to do with the boss but never had the time or energy for was to make huge nuclear warheads pop out of the holes in its shoulders. They would fly out diagonally, off screen, then rain down on the player. (Shadows would tell the player how to avoid it, in classic game trope fashion.) Maybe for future versions?