Most easily done in a solo duel on easy.
1) Acquire
Jace, Vryn's Prodigy from an Origins booster pack.
2) Get him onto the battlefield and transform him into
Jace, Telepath Unbound.
3) Use his +1 ability to build his devotion to 6.
4) Use his -3 ability twice (on two turns) to cast an instant or sorcery in your graveyard.
Note that casting an instant/sorcery from your graveyard removes it from the game so you'll need two different spell cards there. Also be sure to have mana available to cast each spell.
5) Enjoy your 20 gamerscore.