The Treachery was a timely draw, because it provides the perfect opportunity for you to finally cast your Day of Judgment. While the sweeper will still leave your opponent with a few creatures, stealing his Umbra Mystic will help make sure that you can keep pace ― and in this case, get past a certain indestructible blocker.

1. Tap three plains to add WWW, and three islands and Glacial Fortress to add UUUU (three used as generic mana) to your mana pool.
2. Sacrifice the Kithkin Armor to activate its ability, preventing damage that would be dealt to your Scroll Thief this turn. Because you now control no enchantments, you are forced to sacrifice your Tethered Griffin.
3. In response to Kithkin Armor’s ability, Jacopo may tap his Celestial Colonnade for mana and activate Burden of Guilt’s ability. If he does, simply activate your Nomad Mythmaker’s ability in response.
4. Spend W and tap Nomad Mythmaker to activate its ability; return Kithkin Armor from your graveyard to the battlefield attached to your Griffin Rider.
5. Spend 3UU to cast Treachery on Jacopo’s Umbra Mystic. You gain control of the Mystic, and untap five lands: Choose to untap three plains and two islands.
6. Tap three plains to add WWW (all used as generic mana) and two islands to add UU to your mana pool.
7. Spend 1U to cast Aura Graft on Jacopo’s Griffin Guide. You gain control of the Guide. Choose not to attach it to a new creature.
8. Spend 2WW to cast Day of Judgment. This destroys Jacopo’s Shaman en-Kor, Nimbus Naiad, and Daxos of Meletis. This also destroys your Scroll Thief. Jacopo’s Manor Gargoyle survives, because it has indestructible.
9. When Day of Judgment would destroy Griffin Rider, Nomad Mythmaker and Umbra Mystic, you instead destroy an Aura attached to each of those creatures. This destroys Jacopo’s Pacifism and Burden of Guilt, and your Treachery.
10. When Treachery is destroyed, Umbra Mystic returns to Jacopo’s control.
11. When Shaman en-Kor is destroyed, the Griffin Guide triggers. Since you now control Griffin Guide, you put a 2/2 Griffin token onto the battlefield under your control. Your Griffin Rider is now a 4/4 creature with flying.
12. If Jacopo used Burden of Guilt to tap Nomad Mythmaker earlier, he is unable to activate Manor Gargoyle’s ability to give it flying.
13. Declare the start of your combat phase, assigning your Griffin Rider to attack.
14. Jacopo is unable to block Griffin Rider (either because he is unable to activate Manor Gargoyle, or because the Rider has Kithkin Armor attached). Griffin Rider deals 4 damage to Jacopo, defeating him.

UPDATE (15/04/2015): We’ve revised this solution to take Anonycat’s excellent points into account.