I love Mass Polymorph! When I get enough funds, a G/U mass polymorph deck is my ultimate endgoal as well. There's 1 problem I see with this deck. How do you plan on surviving until turn 5+ to get the mass polymorph out? Currently the 3 way I see you doing this are 1) blocking with your tokens from khalni garden (only source on T1-T2), then awakening zone and growth spasm. 2) countering key spells with mana leak. 3) generate bulkier tokens with garruk I recommend pumping up garruk and awakening zone to 4x and swapping some of the deck reorganization spells (remember, they cost mana too) for more removal/ counterspells (i really like spell pierce). This should allow you to survive longer against aggro and red decks as well as provide an alternate (albeit shoddier) win condition of using Garruk and tokens to overrun your opponent in case you don't draw the mass polymorph. If you're really brave, you can even add in a card called next invader. Since he summons a token in with him, he doesn't detract from mass polymorphing a fatty as much as other cards and is a good chump blocker. Also, I dislike explore. Cultivate is probably better since you'll get a card advantage. Finally. I don't really see a reason of having 2 stormtides. I personally really like Iona since she'll single handedly seal the game for you if you get her (or at least cut their efficiency 1/2). I would use two of her instead. True if you polymorph in 2 of them she dies, but its good insurance incase you draw one.
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so im building a somewhat similar deck just without the landfall element. Biggest problem is turn 4 wrath of god on me. I'm also deathly afraid of pyroclasm in turns 1-3. Also, you might consider nest invader. He's like basically a 2/2 for 1 mana with some added versatility. Finally, the only win condition i see is overwhelming stampede, garruk wildspeaker is a much more reliable and versatile option than that imo.