252 SpA Life Orb Deoxys-A Dark Pulse vs. 252 HP / 216+ SpD Arceus-Psychic: 190-224 (42.79 - 50.45%) -- 1.17% chance to 2HKO
252+ SpA Life Orb Deoxys-A Dark Pulse vs. 252 HP / 216+ SpD Arceus-Psychic: 208-247 (46.84 - 55.63%) -- 74.22% chance to 2HKO
4 Atk Life Orb Deoxys-A ExtremeSpeed vs. 252 HP / 40 Def Arceus-Psychic: 104-123 (23.42 - 27.7%) -- possible 4HKO
OR
252 SpA Life Orb Deoxys-A Dark Pulse vs. 252 HP / 208+ SpD Arceus-Psychic: 190-226 (42.79 - 50.9%) -- 3.91% chance to 2HKO
252+ SpA Life Orb Deoxys-A Dark Pulse vs. 252 HP / 208+ SpD Arceus-Psychic: 211-250 (47.52 - 56.3%) -- 82.42% chance to 2HKO
4 Atk Life Orb Deoxys-A ExtremeSpeed vs. 252 HP / 0 Def Arceus-Psychic: 108-127 (24.32 - 28.6%) -- possible 4HKO
Remember Mr.lol that this isn't standard Utility Counter Arceus-Psychic, it's Specially Defensive Utility Counter Arceus-Psychic (Elaboration in the quick summary; the EVs are 252 HP / 40 Def / 216+ SpD OR 252 HP / 48 Atk / 208+ SpD - remember He is a UTILITY COUNTER). Deoxys-A's Life Orb boosted ExtremeSpeed is too weak to successfully KO Arceus-Psychic after SR + Dark Pulse, regardless of Deoxys-A's nature. My post was also assuming "standard conditions" for the counter definition (AKA He doesn't have to deal with SR + additional layer(s) of Spikes, just SR), a definition which Specially Defensive Utility Counter Arceus-Psychic coincidentally fulfills (and exceeds). Is it possible to edit your post in light of this to make it a little more accurate?
Anyway, thanks for placing Arceus-Psychic in the Checks / Counters section :). Got to give credit when it's due, especially when it is Arceus-Psychic!
P.S. Like I said in Post #12, Ice Beam's main advantage over Dark Pulse is its ability to inflict consistent Ice-type damage, as well as damaging Specially Defensive Giratina / Great Wall Lugia / Latias more - essentially in agreement with what you wrote in Post #15. Remember, my intent is to provide both the advantages and disadvantages of Ice Beam / Dark Pulse - nothing more, nothing less. It's up to you to decide on the ultimate slash order based on the facts, experience, etc. (Though it helps to take into account the community's opinion like you are currently doing). Hopefully this makes my position clear Mr.lol - you can choose to edit your post in light of this clarification (EX: State your case to the community, personal experiences, etc.).
On another note, like I said in Post #12, Specially Defensive Latias dislikes the extremely rare (and AC) Ice Punch greatly as she can't switch in safely. She is still one of the better answers to Deoxys-A though with the possible exception of Arceus-Psychic. Is it possible to specifically mention her in addition to Latias as a whole rather than just mentioning Latias as a whole? Likewise, is it also possible to specifically mention the relevant Latias in the AC of Tornadus-T like the other "main" checks? Thanks.