How about some much bigger version of a current enemy? Then he could move really slowly and have one or two long range attacks and a stomp style attack if you get too close. Hmm, might be too easy so lets give him waves of minions to make it more difficult. Also needs to have a really obvious weak spot but otherwise is just a massive bullet-sponge and only challenging because of the high health and minions. Good idea? Oh wait, I just described every boss and sub-boss in the game. Can't wait for DLC!