The description pretty much says it all. Note that you don't need a single creature that big, but rather the total power among all creatures you control must be seventy-five or greater. As anyone who has ever played around with the Elf deck can tell you, this is pretty easy to pull off. To make this as quick and easy as possible, go into Custom Games and create a match against Garruk. I use Garruk because he doesn't have any creature removal for you to worry about and the deck is generally pretty weak when used by the AI.
Set his starting hand size to five and yours to nine. Also set your life total to 40 if you think you need it, but you probably won't. Go into the game and mulligan until you get a decent starting hand, preferably with something to create tokens and/or Immaculate Magistrate. Then just keep buffing creatures and creating tokens without attacking until the achievement pops, then either exit the game or attack for the win.