Go to "Challenges", and select the very last one, Sarkhan Vol. Wait forever until it loads.Your first action will be to play the FREE Summoner's Pact, pulling Eternal Witness from the graveyard. After she's in your hand, you're going to cast Harrow, sacrificing a tapped mountain or plains, and pulling out a forest and a plains. Repeat with the second Harrow, pulling the other forest and plains from the deck. Now, you'll need to activate Khalani Heart Expedition, which will have four counters, and allow you to pull two more lands. Then, use the Elvish Piper's ability, paying one green mana to put Eternal Witness onto the battlefield. With her ability, you'll take back one of those recently cast Harrow spells. Finally, recast the Harrow, sacrificing whatever land you want, and pull your last two lands from the deck, thereby taking eight lands in total, and a completed achievement.
Please see my video below, as proof.