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If I have a {Nivmagus Elemental} on the board, and then I cast a Storm card (let's say {Grapeshot}) can {Nivmagus Elemental} exile the copies of {Grapeshot} to pump itself?

Can {Gaea's Herald} be countered?  Sorry if this is a dumb question, I'm just really not sure.

So in Commander, you can only play cards that are the same color identity as your commander.  Does that mean that in a, let's say, {Elesh Norn} deck, you can't {Figure of Destiny} or a {Boros Garrison}?

No

Holy crap this is awesome!

{Pili-Pala}, enchanted with {Presence of Gond}, and an {Ashnod's Altar} in play. Tap {Pili-Pala} for an elf, sacrifice the elf for {2}, use that to untap {Pili-Pala} and get an any-colored mana.  Repeat for infinite mana of any color! :)

{Ajani Vengeant}+{Tyrant of Discord}
Use Ajani's -7, then play the Tyrant. Of course in Boros colors there's things like {Wrath of God}+{Boros Charm}, but I think his is more fun.

{Eater of the Dead} + {Phenax, God of Deception} = tap the Eater, a player mills 4, hopefully mill a creature, exile a creature and untap, tap again and hopefully mill the whole deck.

I just played  {Heart of Light} on  {Palace Guard} :-0

{Ghostly Possesion} is better; grants flying, so it can block ALMOST anything :)

{Contested Warzone} + {Homeward Path}. Never loose the Warzone!

Sooooo I'm guessing you won that turn?

{Rift Bolt} suspended, THEN {Worldfire}

{Koth of the Hammer} can give you a decent amount of mana for one turn. No infinites that I can think of

Nasty

{Mind over Matter} and {Temple Bell}. I think that's it. If your opponents have a smaller library than you, YOU WIN!!! Sorry if this has been done recently and I missed it