Moves
Quiver Dance allows Xerneas to boost its Special Attack, Special Defense, and Speed, allowing it to sweep opposing teams. Additionally, the Special Defense boost allows Xerneas to more reliably set up as well as effectively handle opposing Imposter Pokemon. Spore and Lovely Kiss are used to put foes to sleep and provide Xerneas with more opportunities to set up as well as grant it extra turns of Poison Heal recovery. Sleep is especially effective against RegenVest users, as they will be unable to pivot or cause damage, and faster Pokemon that Xerneas is unable to OHKO, granting Xerneas with either additional opportunities to attack or to set up. Sleep also allows Xerneas to handle Imposter users by granting it additional turns to damage them and preventing it from being put to sleep itself. Spore is generally recommended over Lovely Kiss due to its 100% accuracy; however, Lovely Kiss is still a viable option to run, as it is not blocked by Safety Goggles. Xerneas has a powerful STAB option in Moonblast, allowing it to hit important Pokemon such as Giratina and defensive Dark-types for super effective damage. It also provides strong neutral coverage towards a majority of the metagame. Lava Plume is useful to run on Xerneas, as it is able to cover opposing Steel-type Pokemon. Additionally, it is able to spread burns to passively damage foes, which can be especially helpful against Pokemon with Magic Bounce, such as Mega Audino, which are unable to be put to sleep, as well as against Shedinja, as a burn would KO it. Magma Storm is an alternative to Lava Plume, as it is still able to provide Xerneas with a strong move to damage opposing Steel-type Pokemon, in addition to partially trapping foes, which can be especially helpful against specially defensive foes such as Mega Audino and Imposter Chansey, as they could be passive and provide opportunities for Xerneas to use Quiver Dance while being unable to switch to a Pokemon that would have a better matchup against Xerneas. This move is also able to deal additional chip damage, which can be very helpful against an opposing Shedinja. Earth Power is a weaker alternative that is able to damage both Mega Gengar and Primal Groudon for more damage, as well as be able to damage Flash Fire Steel-type Pokemon; however, it would also leave Xerneas unable to damage Pokemon such as Celesteela.
Set Details
A Timid nature maximizes Speed and allows Xerneas to outspeed Pokemon such as Adamant Regigigas, Modest Mega Diancie, Timid Primal Kyogre, and positive Speed nature Primal Groudon. A Modest nature is an alternative that provides Xerneas with additional power, allowing it to nearly 2HKO Giratina unboosted. Poison Heal alongside a Toxic Orb is necessary, with Toxic Orb being the most efficient way to guarantee that Poison Heal will activate. Poison Heal is very important, as it makes Xerneas immune to sleep, allows it to beat a foe with Imposter by letting it recover while the foe cannot, and makes Xerneas very difficult to wear down.
Usage Tips
Before Toxic Orb has activated, Xerneas should be used very carefully, as if it does not get poisoned, it loses a great deal of use. An effective way to reliably activate it would be to use a slow Pokemon to pivot Xerneas in, as it will generally be able to safely activate the Orb. If Poison Heal has already been activated, Xerneas is able to provide a beneficial defensive role as a switch-in to Knock Off, as it will not take much damage or care about losing its item. It is also able to work well as a sponge to status effects and can often switch into both Giratina and Mega Audino safely. It is recommended to scout for Pokemon with Unaware, as they occasionally can run sets to nullify Poison Heal, by using moves such as Gastro Acid and Entrainment. Thanks to its passive recovery, good defensive typing, and good bulk, Xerneas is able to be an effective defensive Pokemon against threats such as Mega Tyranitar and Mega Gyarados. Additionally, Xerneas can also fill an offensive role by setting up against Pokemon that are unable to do much against it, such as defensive Yveltal and Mega Audino. However, Xerneas should be careful when facing defensive Steel-type pokemon, as they are often able to survive its attacks, and will occasionally run Flash Fire, and can threaten it with either STAB moves or moves such as Entrainment. Also, be wary of letting Xerneas's HP get too low, as this might leave it unable to effectively KO an Imposter Pokemon.
Team Options
Xerneas appreciates teammates such as Mega Mewtwo Y, which is able to handle faster Pokemon such as Mega Gengar, which could otherwise OHKO Xerneas. It also appreciates Xerneas's ability to break through its checks such as RegenVest Primal Kyogre in return. Xerneas also appreciates teammates that are able to remove opposing Steel-types or significantly weaken them, such as Mega Mewtwo X. Pokemon that appreciate Xerneas's resistance to Dark-type moves or Knock Off, such as Giratina and Mega Mewtwo Y, are also good options for teammates. An Imposter user can scout for Unaware, Flash Fire, or a move that is able to remove Poison Heal, such as Worry Seed, Gastro Acid, or Entrainment. A defensive Steel-type switch-in could be beneficial in case the opponent has their own Poison Heal Pokemon that Xerneas would be unable to defeat, such as Primal Kyogre or Regigigas. If Xerneas is using Spore, it appreciates having a teammate with Knock Off for removing opposing Pokemon's Safety Goggles for Xerneas.