Since Etrigan is not a demonic evil that actual Fallen in the dresdenverse are, I'd just make him a powerful NeverNever being. Jason Blood would just have "Human Form" and my special "Plot Armor" power so he does not die by a high-caliber sniper round to the head from a mile away. Comic characters never seem to suffer from that, with the sole exception of Captain America. Here's what I have for him;
[-3] Plot Armor - Jason Blood/Etrigan has physical immunity to any attacks that would actually kill him directly or indirectly. Treat such attacks as maneuers; somehow, they always tend to just knock him out temporarily, miss him by inches, cause just a flesh wound or otherwise fail to kill him. Any attacks that cause stress but not enough to kill work normally. Any attacks that would take him out but not kill him also work normally.
[+1] Human Form affecting all the following powers;
[-4] Mythic Toughness with Catch of "artifacts of faith and holy powers"
[-2] Nigh Unstoppable - Etrigan may use his might skill as defense against both physical and mental attacks and maneuers. He has a further +2 to might rolls defending against magical attacks.
[-6] Mythic Strength
[-3] Reaper's Claws - Etrigan can do attacks, blocks and maneuers with his claws at Weapons+2. They are Weapon 4.
[-1] Wings
[-4] Hellfire
[-4] Refinement - for Hellfire. Etrigan gets Fire control +2, Fire power +1. The bracers that he wears in his demon form are a +4 offensive power focus and a +4 offensive control focus (including the base item slots from Hellfire)
[-1] Spellshear - Etrigan gets +2 to attack rolls against magic - such as magical blocks and magic armor or other protective magic.
+7 Might, Conviction
+6 Weapons, Discipline, Athletics
+5 Endurance, Presence, Intimidate
+4 Investigation, Alertness, Lore
Most other skills default to good, except for high-tech skills that default to average.