Go to the first challenge against Garruk. You must win in your only turn to complete the challenge.
The trick is easy to this one. Use both of your Angelic Blessings on your Skyhunter Skirmisher, making it a 7/7 with flying and double strike. It will move you to your attack phase and attack with it. Garruk cannot defend and the Skyhunter will attack for 14 damage, defeating him.