Sarkhan's final deck is is built around his Eldrazi (huge endgame creatures) and dragons using Solemn Simulacrum to accelerate his land drops and several removal spells to hold off until he has the required mana to play these massive creatures. Another card to look out for and which must be dealt with before its effect kicks in is Dragonmaster Outcast. The best counter would be a quick kill deck with several flying creatures and removal spells so the Relics of Doom deck would be the ideal choice, but again use whatever deck you feel comfortable using. You can also reduce the difficulty to easy if needed and don't be afraid to mulligan or reset to get a good opening hand. Once you defeat Sarkhan's final deck, you'll unlock the achievement and Sorin Markov's vampire deck, Master of Shadows.