First off you have to get Nemesis of Mortals on the battlefield. Nemesis is a green creature card with a casting cost of 6 mana, two of which must be green. Then you have to activate his monstrosity ability which costs 9 mana, two of which must be green. The monstrosity cost is lowered however by 1 mana for every creature in your graveyard. The lowest cost possible is two green mana as only the generic cost is reduced. All you have to do is keep your nemesis alive until you have at least 7 other creatures in your graveyard. Doesn't matter how they get there. You can chump block with small creatures and have them die, discard them or send them to the graveyard directly from your library. As long as you have 7 or more creatures in your graveyard the monstrosity cost will be two green. Click on your nemesis and pay the two green and the achievement is yours.
Super easy with a boosting partner. Nobody attack each other. Just keep drawing cards and only playing lands until you get your nemesis. Cast him and nothing else. If you have more than 7 cards at the end of your turn will have to discard down to 7. Discard all creature cards other than nemesis. After the 7th creature is discarded (likely before you have even drawn the nemesis) you are free to pay the double green and the achievement will pop.
Alternatively, you can build two decks. 1 with only forest and green creatures (making sure Nemesis is among them) and a 2nd deck that is sabotaged. 30 plains and 30 black cards. Your opponent will not be able to cast anything the entire game. Play the green deck yourself in a practice duel and set the AI to the sabotaged deck. Play 4 forests only. Just keep discarding creature cards at the end of your turn till you have at least 7 in the graveyard and a Nemesis in your hand. The Nemesis cost will then be 2 green and his monstrosity ability will be 2 green as well.