Only card idea for me that comes to mind would be "The Exit is This Way," with solid walls of bullets fired down from either side of the boss, leaving a safe spot directly below as they move around (a good bit, to keep it interesting). Plus some streamed/random crap to keep it nontrivial.
Well, that'd be the spellcard version. Plain would just be vertical lines of bullets with the same placement scheme. (Sort of like Youmu's PoFV L2/L3 or something, come to think of it).