Thanks to Extreme Killer's very good bulk and the priority of His Extreme Speed, He is very difficult to revenge kill. Combine it with a base 120 Attack stat, the ease of setting up a Swords Dance, and STAB on said Extreme Speed, and you get one of the best late-game sweepers in the entire game, capable of decimating offensive teams and cleaning up weakened defensive teams with ease.
Swords Dance will enhance the power of His STAB Extreme Speed to incredible levels. With just a Silk Scarf and Swords Dance boost, Extreme Speed is more than capable enough to OHKO even the deadliest of Uber titans, such as Kyogre, Kyurem-W, Mewtwo, Latias, Palkia, and Reshiram.
To repel the Ghost-types that can come in on Extreme Speed, Shadow Claw is the preferred option in the third slot for its high reliability. For those that crave more power, Shadow Force is another option. Its power after a Swords Dance is so extreme that even physically defensive Giratina risks being OHKOed without any hazards provided Arceus is Adamant and holding Life Orb; Giratina-O, Arceus-Ghost, and Wobbuffet are easily OHKOed. Shadow Force also evades crippling support moves such as Will-O-Wisp during its charge turn, though the charge turn in itself can prove most inconvenient against Lugia and Normal-type Pokemon. For His last slot, Earthquake eradicates the Rock- and Steel-types that could otherwise take an Extreme Speed, such as Dialga or Heatran. Brick Break, though less powerful, breaks through troublesome screens and is significantly more effective against Bronzong, Ferrothorn, Skarmory, and Wallceus. Recover improves His longevity and allows Him to act as a pivot at the detriment of coverage.
With STAB Extreme Speed as His judgment, one might wonder why Arceus even bothers with Speed. After all, the given EVs and nature grant Him a superb combination of maximum power, bulk, and just enough Speed to outpace neutral base 95s. But alas, Arceus's incredible versatility allows Him to run several EV spreads just as effective as the last. Invest as many Speed EVs as desired to outspeed certain key threats. To outspeed positive base 90s like Timid Dialga, 124 Speed EVs is sufficient. 168 Speed EVs will outspeed any positive base 95s like Jolly Rayquaza. For other key threats, a Jolly nature becomes necessary. 160 Speed EVs is sufficient for outspeeding Terrakion. 176 Spe EVs will outpace positive base 110s like Froslass. Finally, 252 Speed EVs will guarantee a Speed tie against maximum Speed Arceus, which makes them unreliable answers, especially if they're using Will-O-Wisp. Once the desired Speed has been reached, maximize Attack and then place any remaining EVs into HP to improve His overall bulk. However, do recall that the greater the Speed investment, the lesser the bulk.
Unlike His other formes, Extreme Killer can utilize almost any item. For those seeking a balanced approach, Silk Scarf is the preferred option. It provides a 20% boost to Extreme Speed without compromising bulk. For a more offensive touch, Life Orb provides a great deal more power to all of Arceus's attacks, though the recoil damage is unfortunate. With just a Life Orb and Swords Dance boost, Extreme Speed is capable of OHKOing threats such as Kyurem-B, Zekrom, and tank Ho-Oh without any hazards. It is also the best item to use against His top defensive checks. If the Speed of Jolly is favored, Life Orb is recommended to compensate for the loss of power in comparison with Adamant. For those that favor one-time status protection, Lum Berry is the best option. It is particularly useful for its utility in lead positions and SmashPass situations. However, the power loss is substantial: without the boost from Silk Scarf or Life Orb, Arceus is unable to OHKO many Pokemon like Palkia and Mewtwo without sufficient entry hazard support. For those that wish to maximize Arceus's staying power, Leftovers takes full advantage of His bulk. It works well with a bulkier spread in combination with Recover, dropping Attack to make room for more defensive EVs. Power Herb can be used alongside Shadow Force and Stealth Rock support to always get past Lugia and specially defensive Giratina, but is very situational.
Although the given coverage moves are generally the most reliable, these slots are extremely flexible. A Life Orb-boosted Overheat incinerates Skarmory, Sableye, and Bronzong under sun, but is next to useless under rain. If Life Orb is forgone, Fire Blast is a better option while also having increased utility against Ferrothorn and Forretress. Dragon Claw can be used over Shadow Claw for its increased power against Giratina and Giratina-O, but loses utility against Lugia and Arceus-Ghost. Grass Knot 2HKOes Groudon and Hippowdon on the switch, but restricts coverage substantially. Similarly, Safeguard can prevent status for multiple turns, while Substitute eases prediction, protects against status, and punishes revenge killers.
Two approaches exist to counteract Arceus: defensive titans and revenge killers. Defensive titans consist of Toxic stallers, phazers, or Will-O-Wisp abusers like Skarmory and Wallceus; revenge killers consist of Pokemon like Terrakion that can survive one of His faster boosted attacks comfortably yet deal considerable damage back. Arceus is specialized: He might be able to overpower several checks with a given moveset, but He is unable to overpower everything. As such, against defensive titans, the best support is to weaken any defensive Pokemon capable of dealing with His specialization beforehand. Luring them out and sufficiently damaging them will make the most of His sweeping abilities. Pokemon such as Blaziken, Groudon, Ho-Oh, and Rayquaza can make excellent lures, as Arceus's defensive answers are often used in response to these threats. Powerful wallbreakers such as Kyogre, Kyurem-W, Latios, Mewtwo, and Reshiram can blast holes through the opponent's team, possibly exposing these Pokemon as well as easing Arceus's sweep. Entry hazard shuffling is also beneficial in both the damage and exposure element. Finally, specific answers that can come in on troublesome defensive Pokemon happen to be excellent teammates. Magnezone can trap and eliminate Skarmory. Xatu and stallbreaker Heatran are excellent answers against Wallceus. Giratina and Lugia dislike Darkrai, Zekrom, Reshiram, and Kyurem-W.
Regarding revenge killers, the best support is to weaken them to the point where a faster boosted attack can OHKO, cripple their Speed, or reduce their offense to a manageable level. Entry hazards are very helpful for their damaging properties; Pokemon such as Deoxys-S, Ferrothorn, and Groudon are excellent entry hazard users. Lures such as Swords Dance Rayquaza or double status Darkrai can either deal considerable damage or cripple with status. Proper weather control through Pokemon such as Kyogre and Groudon is crucial as several revenge killers, such as Kabutops, Omastar, and Choice Scarf Heatran, are dependent on it. Dual screens from Pokemon like Deoxys-S and Latios can be helpful for softening enemy assaults. Carrying specific answers to His revenge killers is also a wise choice. For instance, Wobbuffet can dispatch Choice Scarf users while also buying Him a turn to use Swords Dance if necessary. Of course, keeping Arceus healthy is an option as most revenge killers are unable to OHKO Him should He be at relatively high health.