So, after making the graphic that I emailed you, I was getting ideas like... He could summon enemies like you mentioned, but while you're running around killing those enemies, he could be shooting something in your direction that you have to be dodging simultaneously.
He could also use the scythe on his back, but rather than typical slashing like you would expect, he would be moving and slashing it with some sort of telekinesis, which the player would have to dodge. Then at times the boss would physical walk towards the player with the scythe in hand and try to slash him, at which point the player would have an open shot for melee.
So, I think the scythe should be fairly simple. It should probably fly down to the level that the player is at and quickly sweep across the stage, pause, sweep back, and then fly back into the boss' hand. This will most likely be implemented as a decal with some pretty complex AI. I also have an idea how to make a motion blur / trail which I'll be experimenting with.
The projectiles (or whatever he's shooting at you) will be a special case; they probably won't update like regular projectiles, since we'll need them to travel on a specific angle towards the player.
I thought it would be cool if it was evident that he was gradually weakening from the strain of summoning enemies and controlling his scythe, so after each wave or attack he'd be hovering a little lower. Once he was drained low enough he would be at a height that it would be easy to get to him. At this point he'd have to resort to swinging at you with a melee attack.
What do you think? Any of this can be changed, obviously. I'd be happy to take on the programming aspect of it if you want.
That sounds perfect. I think I'll have him shoot red lightning bolts, like what you see coming from the sky.
I need to get the sound effects and music done (not to mention the Audio "engine" implemented), so, if you don't mind, I think you'd should take the coding of it. I'd be happy to help once the other stuff is done, though.