Pokmon ex rule: When your Pokmon ex is Knocked Out, your opponent takes 2 Prize cards
Disappear triggers on your end step and looks back at your entire turn
While that's not the most terrible idea, the likelihood in a Niv-Mizzet deck is that the other spell you're casting is a burn spell that could do the same, and I have to feel that you're going to be better off casting it by itself, rather than putting all your eggs in one basket
If youve been to a Target, Walmart, or other large store lately, you might have seen some Disney Lorcana card packs next to all the Pokmon and Magic cards
This means that they can attack and use activated abilities with the tap symbol on the same turn they enter the battlefield
Thing is, for nine mana you do want the game over, not in a position where you are ahead or rely on getting some luck