Admiral Pierce is the first "boss" of the game.
You will begin on the start learning the controls. The game will explain how to command the troops, how to avoid the fire, and in the last screen you will reach the outside of the ship, where Admiral is swinging his rope trying to avoid you and your minions. Just avoid pressing
when he is jumping and when he lands, go for it.
It is a pretty easy boss so not hard to defeat, unmissable if you play the story.
BTW: I have to say that unlocking achievements on this game was very buggy until version 1.1.0c (you see it on the bottom right part of the screen when the game launches / main menu).
I almost finished the game on released version and got almost no cheevo. Game anyway resetted my progress (it happens also on PC) so I had to start again.