You don't need the "he or she controls" clause (see Braids, Cabal Minion), as that's built into the definition of sacrifice. And although this is clearly a powerful effect, I think the stats could be a bit higher on a mythic.

How's that look?

Yeah, that looks more like a mythic Wizards would print.

• Is first strike necessary? I would just cast him, and keep making everyone sac creatures until I was the only one with any left. Then swing. I wouldn't attack with another creature on the board.
• Maybe Legendary?
• Maybe add a tap to that so someone doesn't abuse it. Next turn everyone's sacrificing 2 creatures. Then swing.

Well, he hits everyone including you so if you are behind on creatures that won't work. First Strike represents that he can still deal death up close and personal as well.