Heh, this one took me a LONG time to figure out as well. Eventually, I figured out that Ebon Dust Aura works well to negate most of his damage. It's been a while since I've done it, but I think this is how the main part went:
[skill]Victorious Sweep[/skill][skill]Mystic Sweep[/skill][skill]Flail[/skill][skill]Ebon Dust Aura[/skill][skill]Staggering Force[/skill]
Scythe Mastery: 12+1+1
Earth Prayers: 12+1
Mysticism: 3
To the left side of where you arrive, there is a building with 2 little spike things jutting from the ground in front of it. Go there and wait for the Griffon to arrive and cast it's spirits. Once the Griffon is done, pull it next to you and if you do it right, the spirit's attacks shouldn't hit you. Immediately hit Mystic Sweep to blind him and start auto-attacking. Activate Flail whenever it's up and use Victorious followed by Mystic Sweep to take out any spirits that the Griffon then tries to summon. This takes a few tries to get the hang of and relies on a bit of luck in pulling the Griffon.
Also, a variant would be to use Heart of Fury instead of Flail and bring Mending Touch or Dismiss Condition to remove blind if Shadowsong manages to hit you. If this is the case, time your condition removal to land just after Shadowsong hits you, then immediately hit Mystic Sweep to (hopefully) dispatch it and continue on. Also, put the attributes to:
Scythe Mastery: 11+1+1
Mysticism: 11+1
Earth Prayers: 8+1
I hope this helps you

.
(P.S. This general build also dispatches a lot of the hard bosses in EotN such as Magni the Bison)