Moves

• Giratina-O's ability to force switches with its strong attacks and check Primal Groudon makes it an effective Defog user.
• Hex provides Giratina-O with an immensely powerful attack against statused foes. Hex is one of the major factors that contribute to Giratina-O's offensive presence due to it 2HKOing foes like Primal Groudon, Primal Kyogre, Magearna, and the majority of the support Arceus formes once they've been statused.
• Dragon Pulse is a consistent secondary STAB move that isn't reliant on status. Dragon Pulse also has a sizable damage output despite its mediocre Base Power, doing enough damage to 2HKO most offensive Primal Groudon variants after Stealth Rock damage and outright OHKO offensive Mega Salamence.
• Draco Meteor provides Giratina-O with a Dragon-type STAB attack with more immediate damage output, forcing recovery moves from support Arceus formes that attempt to absorb damage and threatening to OHKO Yveltal and offensive Ho-Oh after Stealth Rock damage. Draco Meteor also pressures Primal Groudon significantly harder than Dragon Pulse, weakening defensive variants to the point where it can no longer pivot into threats like Necrozma-DM and Xerneas and putting offensive variants in range of priority moves. However, its tendency to force Giratina-O out after attacking a foe and worsen its matchup against Substitute Extreme Killer Arceus makes it a less favorable move than Dragon Pulse.
• A status move is used in the fourth moveslot to enable Giratina-O to use boosted Hex and punish a variety of its switch-ins. Thunder Wave cripples support Arceus formes, Yveltal, and Xerneas, making them much easier to check. Paralysis allows Giratina-O to threaten support Arceus formes by threatening to 2HKO them with boosted Hex, and it comes in handy with punishing Xerneas, forcing Choice Scarf variants to lock themselves into Aromatherapy and making Geomancy variants significantly easier to handle. Thunder Wave especially comes in handy where Giratina-O isn't healthy enough to win duels against foes like Mega Lucario, enabling its teammates to handle them more easily.
• Will-O-Wisp is an alternative status move that pressures Steel-types harder with residual damage and cripples Necrozma-DM, Mega Lucario, and Swords Dance Arceus formes.
• Toxic can be used to pressure Primal Groudon and punish Giratina-O's checks harder in the long run, serving as the only status move that can pressure support and Calm Mind variants of Arceus-Ground, which would otherwise not mind too much about Giratina-O's other moves. However, choosing Toxic as a status move removes Giratina-O's ability to 2HKO specially defensive Necrozma-DM, and it is usually better for Giratina-O to use Thunder Wave or Will-O-Wisp and let its teammates spread Toxic poison to foes like Primal Groudon and support Arceus formes.
• Giratina-O can forgo Hex for Shadow Ball and give up a status move for Shadow Sneak to have a consistent Ghost-type STAB attack and a priority move, which can be handy with picking off faster threats like Mega Gengar and Deoxys-A and setup sweepers. A combination of Shadow Ball and Shadow Sneak is also capable of preventing Cloyster from setting up entry hazards. Note that Giratina-O will have to opt for a Quiet nature with this option to maximize Shadow Sneak's damage.

Set Details

• 248 HP and 52 Special Defense EVs allow Giratina-O to survive two full-powered Eruptions from Primal Groudon.
• The remaining EVs are put into Special Attack with a Modest nature, which, combined with a mandated Griseous Orb, improve Giratina-O's offensive presence, limiting its switch-ins and enabling it to offensively check a large portion of the tier.
• If Thunder Wave is run, 28 Speed EVs should be used to outspeed Modest Choice Scarf Xerneas and Yveltal once they have been paralyzed.
• Running 152 Speed EVs may be an appealing option if using Will-O-Wisp, as this enables Giratina-O to burn Necrozma-DM before Necrozma-DM OHKOes Giratina-O with +2 Searing Sunraze Smash. Note that this costs a noticeable portion of Giratina-O's damage output or bulk, however.
• Maximum Defense investment can be used instead of maximum HP investment to take physical attacks better. This can come in handy against Pokemon like Necrozma-DM and Swords Dance Arceus-Ground.
• Levitate provides Giratina-O with an immunity to Ground, improving its matchup against Primal Groudon and Swords Dance Arceus-Ground.

Usage Tips

• Making the best use of Giratina-O requires somewhat cautious play, despite it often being a pivot that is intentionally sent out against Primal Groudon and certain entry hazard setters. Giratina-O will inevitably sustain damage and will be worn down over the course of the match, and this means Giratina-O should be somewhat focused on managing its HP.
• Giratina-O should be quick to decide whether combating a foe and taking damage outweighs lessening the chance it can take on other threats it may be tasked to handle. Taking damage and removing entry hazards with Defog is beneficial for Giratina-O's team, but staying in the field unnecessarily long to take an extra attack or increase the odds of getting inflicted by status will only make Giratina-O get worn down faster.
• Primal Groudon and Swords Dance Arceus formes are the most common foes that Giratina-O should stay in against to prevent them from dealing excessive damage to other teammates. While these threats may be easier to keep in check with a healthy Giratina-O when they attempt to wallbreak early- or mid-game, it will be much more difficult to stop them as the game gets longer and Giratina-O keeps taking damage. Unless there are reliable checks to said foes other than Giratina-O, ensure that Giratina-O is healthy enough to win the duel against them.
• Against bulkier teams with durable entry hazard setters like defensive Necrozma-DM, Ferrothorn, and Arceus formes using Stealth Rock, Giratina-O can choose to act aggressively by statusing them and then damaging them with Hex. While entry hazard setters from bulky teams tend to carry Toxic, this may be a worthwhile move to make if entry hazards are going to be problematic for Giratina-O's team and the setters have to be pressured.
• Giratina-O will find an opportunity to remove entry hazards from offensive teams if it manages to pivot into an appropriate foe like Primal Groudon or Arceus-Ground. As these Pokemon are common in such archetypes and simply letting Giratina-O switch into them will let it remove entry hazards set by dedicated leads, it is likely that an opponent will seek a way to wear down Giratina-O with Stealth Rock and aggressively double switch into offensive checks like Xerneas and Yveltal, so avoid being too reactive against such teams.

Team Options

• Various support Arceus formes can be beneficial, as, depending on their typing, they can cover Giratina-O's weaknesses and form a good defensive core. With Giratina-O freeing their moveslot by providing Defog support, Arceus formes have more options to be better checks to whichever foe they desire to cover for the team and can cover foes like Zygarde-C and Mega Salamence that will either force Giratina-O to directly combat them to keep them in check or overwhelm it after some prior damage. Arceus-Water is the most notable one, as it can handle most Primal Groudon variants with Giratina-O and can sponge attacks from special variants of Primal Groudon and Ho-Oh, which Giratina-O cannot switch into consistently. Arceus-Dark soft checks Necrozma-DM and Yveltal while benefiting from Giratina-O drawing in and statusing checks like Xerneas.
• Various Steel-types like Necrozma-DM and Ferrothorn can pivot into Toxic and Fairy-type attacks aimed at Giratina-O. Practically any Steel-type appreciates Giratina-O's ability to stand against Primal Groudon and can set entry hazards or spread status in return. Magearna is a notable partner that can pivot into both Xerneas and Yveltal, provide cleric support, and benefit from Giratina-O's Defog support, preventing Spikes from rapidly wearing it down.
• Stealth Rock users like Primal Groudon and Necrozma-DM can prevent defensive Ho-Oh and Yveltal from recklessly switching into Giratina-O and can run Toxic to cripple Defog users and support Arceus formes, giving Giratina-O an easier time to deal with foes that it would need poisoned to deal with.
• Flying-types enjoy Giratina-O's Defog support and can also function as secondary checks to Primal Groudon offensively or defensively. Ho-Oh is one of the Pokemon that benefit the most from the absence of Stealth Rock to be able to pivot into Fairy-types, absorb status moves reasonably well, pressure Steel-types, check some Primal Groudon variants, and spread status to make Giratina-O's Hex easier to utilize. Yveltal offensively checks Ghost-types, cripples special walls with Taunt, and benefits from Giratina-O's ability to leave Arceus formes crippled by status or a large amount of damage they took while combating it. Mega Salamence can absorb status moves for Giratina-O to use boosted Facade and can have a better chance to sweep after its checks like Steel- and Fairy-types have been pressured by Giratina-O.
• Mega Gengar and Giratina-O can work together to spread various status ailments and facilitate their attempts to prey on as many foes with boosted Hex as possible. Mega Gengar can trap and dispose of Fairy-types and special walls, while Giratina-O can pivot into threats like Primal Groudon and Ho-Oh that Mega Gengar struggles to deal with. Trapping and removing Stealth Rock users like Arceus-Fairy that Giratina-O cannot effectively deal with is also appreciated.
• Toxapex can set Toxic Spikes to ease Giratina-O's attempts to damage foes with boosted Hex, take advantage of Arceus-Fairy, and check Marshadow and Ho-Oh. In return, Giratina-O can pivot into Ground-type attacks aimed at Toxapex.
• Cleric support from Xerneas, Chansey, and Blissey can be helpful to prevent Giratina-O from losing too much HP from status. Chansey and Blissey can also support Giratina-O with Wish to further extend its longevity.