"Taking all day" was definitely exaggeration on my part, I apologise. What I mean is that it can take one very comfortably, and burn it or do major damage back, which Giratina and Lugia can't do. Nothing counters Extreme Killer, but Fightceus in my opinion is the strongest check. Lugia can only phaze away, whereas Fightceus and Giratina can phaze AND burn. The difference between the two is that while they are both 2HKO'd by E-Speed or Shadow Claw respectively (Impish Max Defense Giratina takes 52.28% min from LO +2 Shadow Claw), Fightceus can do damage back, whilst Giratina can't (REAL damage anyway). By the way, I never said Fightceus "KO'd it all day", just that it can do significant damage back.This makes him the strongest check to Extreme Killer in the game, or at least top three.
In Ubers, Fighting is better defensive typing than most because it isn't weak to ice or dragon. They are probably the two most common attacking choices, so not being weak to either is a huge boon, something neither Giratina nor Lugia can boast. Flying moves is what, Ho-oh? Ok, granted, but Giratina and Lugia don't appreciate Brave Birds to the face either. Pyschic is Mewtwo basically, who beats Giratina and Lugia one one anyway.
Giratina hardly tanks SpecOgre: 252+ SpA Choice Specs Kyogre Ice Beam vs. 248 HP / 252+ SpD Giratina: 240-284 (47.71 - 56.46%) -- guaranteed 2HKO after Stealth Rock
It can switch in once, and once only, (even if you predict correctly, Water Spout still does 42% min).
What does Lugia do better against E-Killer than Fightceus? Assuming Multiscale is up is too presumptious, most players will never set up with a multiscale Lugia on the field. Even then:
+2 252+ Atk Life Orb Arceus-Normal Shadow Claw vs. 252 HP / 176+ Def Lugia: 260-307 (62.5 - 73.79%) -- guaranteed 2HKO after Stealth Rock
As you can see, Lugia takes the +2 hit the exact same as Fightceus, except it's a move with a higher crit ratio, so surely Fightceus has a tiny advantage here?
Lugia also cannot take Ice Beams from Mewtwo either:
252 SpA Life Orb Mewtwo Ice Beam vs. 252 HP / 80 SpD Lugia: 200-237 (48.07 - 56.97%) -- guaranteed 2HKO after Stealth Rock
I wasn't talking about CM Fightceus btw, the set I'm referring to carries Will-O-Wisp, Ice Beam, Judgment, and a variable last slot between recover, roar, perish song, or whatever you may need. And of course Ghostceus deals with it, that's just a given, in the same way Ho-oh deals with Ferrothorn. Every pokemon has something that deals with it to a degree, but what Fightceus deals with is worth having him being dealt with by Ghostceus.
Fightceus isn't perfect, but the very fact that he does check E-Killer means that he at least warrants a little bit more than 1% usage when E-Killer is on about 30% of all teams. I'm not calling for him to be top ten, but when put in a solid defensive core, Fightceus becomes one of the better Arceus forms.
EDIT: Bold Fightceus isn't actually horrible, if you want it to have a meh chance of living two +2 Espeeds:
+2 252+ Atk Silk Scarf Arceus-Normal ExtremeSpeed vs. 252 HP / 252+ Def Arceus-Fighting: 207-244 (46.62 - 54.95%) -- 66.41% chance to 2HKO
I wouldn't, but it may win matches now and then.