I know the following card was spoiled a few days ago, but I've had this deck idea floating around in my head ever since I saw it:
It seems to me that if this card is every played in constructed, it'll only fall into a very narrow deck type, one that is specifically designed around the Nivmagus. The first trick in breaking it is to treat it like a delver. You want to drop the Nivmagus early, protect it at all costs, and tempo out your opponent as the Nivmagus brings the beats. The second trick in breaking it is to note that you can exile copies of instants and sorceries just as easily as any other spell. Which brings us to a logical conclusion... Storm.
I'm thinking 4 copies of Nivmagus Elemental and 4 copies of Flusterstorm is a good place to start. Imagine dropping the Nivmagus on turn 1 and then protecting it against your opponent's spell with Flusterstorm on turn 2 and imagine them tapping out to do so. You counter their spell with one copy and you exile the excess storm copies to the Nivmagus. Now imagine that storm goes off for 2 or 3 copies and away we go.
We're clearly in the Modern format, so let's throw in some Grapeshot, build up some excess storm, and exile the excess copies to the Nivmagus. How big is he now? 5/6? 7/8? 9/10?
If we're playing casual multiplayer, why not throw in a couple copies of Radiate and let's really have some fun. How about casting Lightning Bolt, targeting it with Radiate, and only exiling all the copies of Lightning Bolt that target yourself or permanents you control. The Nivmagus will grow bigger and 3 damage will be dealt to all players & creatures you don't control.
To go even deeper into the tank, let's throw in some Fire / Ice (also a good Radiate Target) and a pair of Isochron Scepter.
Nivmagus Elemental was born to exile copies of spells. Might as well abuse him to great effect.