Arceus-ghost is weak to Shadow Claw everything Extremekiller uses though while Normal Arceus isn't weak to anything save Brick Break. The reason to use Return over ExtremeSpeed is because it has way more PP, useful for a wall and hits stronger. Also Wall Arceus doesn't need that priority since it will outspeed standard Ho-Oh anyway.
Giratina normal forme doesn't carry Toxic usually. It uses Will-O-Wisp almost ALWAYS all the time for stopping physical sweepers easily, not Toxic. Toxic is used on Lugia more.
252 Atk Life Orb Arceus Shadow Claw vs 252 HP/4 Def Arceus-Ghost: 36.04% - 42.34%
252 +2 Atk Life Orb Arceus Shadow Claw vs 252 HP/4 Def Arceus-Ghost: 72.07% - 84.68%
Survives even a +2 to hit back with Will-O-Wisp
Additionally, Arceus-Ghost has the utility of rapid spin Immunity in addition to a nice Fighting Immunity
I'm not going to say I know more about what every person runs on their Giratina. I will say that I see Toxic as often (and sometimes in conjunction with) Will-o-Wisp.
Ghost Arceus is outsped by both Mewtwo and Darkrai anyway. Also Mewtwo can smack Ghost Arceus with Psystrike or sun-boosted Fire Blast. Darkrai can just pummel you with 100% accurate STAB Dark Pulse, that's worse than against Defensive Normal Arceus lol.
252 SpAtk Life Orb Darkrai Dark Pulse vs 252 HP/0 SpDef Arceus-Ghost: 67.57% - 79.73%
252 SpAtk Life Orb Darkrai Focus Blast vs 252 HP/4 SpDef Arceus: 67.57% - 79.28%
Those look pretty damn similar to me. If Darkrai decides to be Modest (albeit more unlikely than Timid, but hey) then it, in fact does, not outpace the Arceus-Ghost set.
I will like to see how Scarf Terrakion can OHKO Mewtwo outside of a critical hit. Chansey has a chance to survive Jolly Close Combat at full health.
For this: I apologize. I understimated Mewtwo's defenses.
252 Atk Terrakion X-Scissor vs 4 HP/0 Def Mewtwo: 58.76% - 69.49% and Stone Edge does slightly less. (55.08% - 64.69%)
barring crits, a 2HKO
more than likely less against Stall2
however Stall2 doesn't overly threaten ghost-arceus
Chansey however does not fair nearly so well:
To survive you would have to get a free switch with no hazards at essentially full health
252 Atk Terrakion Close Combat vs 4 HP/252 Def Eviolite Chansey: 88.32% - 104.21%
Gets OHKOd by adamant Terrakion (who is faster than Scarf Jolly Zekrom but not Scarf Jolly Garchomp)
Plus as a partner to Arceus-ghost Terrakion can switch in on the resisted Dark Pulse from Darkrai for a nice +1 Attack which changes the damage against mewtwo to 88.14% - 103.95% and 82.2% - 96.89% for X-Scissor and Stone Edge, respectively.
Again, not that a switch from Darkrai to Mewtwo is
likely this is just good to know.