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Niv-Mizzet is a pretty good commander but he's only UR which is rather limited in terms of card selection. Your general suffers the same problem, You lack the 2 big creature control colors W and B. You do have green so you can accelerate faster then an UR deck can and I guess that's what you should focus on.
You could for instance accelerate into broken cards like Primeval Titan, Genesis Wave, Tooth and Nail, Rude Awakening, Avenger of Zendikar etc...

If you open with Cultivate, Sakura-Tribe Elder, Kodama's Reach, Oracle of Mul Daya, Primeval Titan, Far Wanderings, Explosive Vegetation and cards like that... it will be hard to follow your tempo.

If he steels a lot of cards there is both a land (packed in one of the 5 commander decks) and a green creature that says every one gains control of the cards he owns. Green lets you fetch both creatures (Green Sun's Zenith, Worldy Tutor, ...) and land (Primival Titan, Reap and Sow , Sylvan Scrying, Expedition Map)


What you can also do is play a better commander, one that allows W or B and play tons of spot removal to kill his general.

good luck


PS: If you want to be an ass you can go Life From The Loam/Crucible of Worlds/strip mine shenanigans and green cards that say you may play multiple lands each turn. It's no fun but it wins competitive games.