First, go to the deck editor and make sure your deck has plenty of creatures in it. 21-25 would be a good number. I suggest a black and green deck utilizing cards like Satyr Wayfinder and bloodflow connoisseur to beef up your graveyard size and help you win the battle at the same time. When you have the opponent on the ropes and just need one more attack for the win, count the number of creatures in your graveyard. If you have less than 20 stop attacking and sit back. Designate one large creature to stay on the battlefield to keep attacking and taking out the creatures your opponent plays. If using black and green like i suggested, Nemesis of Mortals works amazing with this and even will help you net the "why did it have to be snakes" achievement. Most of the creatures die easily to a Nemesis if you have paid the monstrosity cost to make him a 10/10. If your opponent plays a creature, attack with your large creature on your turn. Even though the AI player is doomed it will always block to keep itself alive a bit longer. When your opponent has an empty battlefield thanks to your Nemesis or other creature, don't attack. Do not even play a land or anything so that if you have a hand of 8+ cards you can discard them at the end of your turn. If you have 8 or more cards in your hand at the end of your turn you are forced to discard down to 7. It is easier to discard the drawn creature cards than it is to play them and hope the enemy can destroy them. (although turning off simplified targeting in the options menu will help you to target your own creatures with destroy effects) This will take most of your entire deck and a bit of time if you dont have a way to speed into your deck drawing extra cards and dumping piles into your graveyard directly from your library. when you finally have 20 or more creatures in the graveyard just attack your opponent for the win.
If you want to cheat this achievement can be obtained super easy. Build two decks. Deck 1 is 24 lands and 36 creatures of the same color. It does NOT matter which creatures or which color. Deck two is 30 forests and 30 red cards. The idea is to make every card in this 2nd deck uncastable. Go to practice game and set the heavy creature deck for yourself and the sabotaged deck for your opponent. Just dont play anything but lands for the first half of the game. When you have 8 cards in your hand at the end of the turn from not playing any creatures you will have to discard down to 7. Only discard creatures this way. After 20 turns discarding creatures you are free to start casting them and attacking at will. The achievement will pop as soon as the match ends.