Actually it's more like I remember my Rayquaza OHKOing his Giratina-O :P and he complained how is Outrage strong enough to OHKO him...
Giratina-O @ Griseous Orb
Trait: Levitate
EVs: 116 HP / 252 Def / 136 SAtk / 4 Spd
Modest Nature (+SAtk, -Atk)
- Calm Mind
- Dragon Pulse
- Rest
- Sleep Talk
How is this? Max Defense allows Giratina-O to survive Scarf Zekrom's Outrage, bulkier than Timid Ghost Arceus physically, Levitate, Modest gives it more power than no SpA Arceus and hits way harder than Giratina, useful for 2HKOing Dialga at +1.