Both Forms of Giratina are powerhouses in Ultra League PvP. The high Defense (for Altered Form) and Stamina (for both Forms) allow Giratina to take significant damage. Although Giratina has five weaknesses, both Ghost and Dragon take increased damage from moves of that same type. Ice on the other hand deal increased damage, but take neutral damage. Lastly, the lack of a Quick Fairy Move, holds Fairy counters back.

Giratina Altered PvP counters

Altered Giratina DragonGhost
Effective Stats at Level 28 (IV 0/14/15)
ATK 187		DEF 225		HP 284
Weak to			Strong against
Ice Ghost Fairy Dragon Dark			Bug Electric Fire Grass Poison Water Fighting Normal x2
Best PvP move set for Altered Giratina
Shadow Claw Ghost		Ancient Power Rock		Dragon Claw Dragon
Super Effective versus
Psychic Ghost		Ice Fire Flying Bug		Dragon
Move Stats
Power 6 Energy 8 Duration 1 DPT 6 EPT 8		Power 50 Energy 35 DPE 1.43		Power 70 Energy 45 DPE 1.56 Dust cost 100000 Candy cost 100

Altered Giratina’s high Defense and Stamina Stats means it will outlast most any counter. Ancient Power’s Rock-typing makes it Super Effective against Ice counters. The key is finding a Pokémon that also has high Defense and Stamina, but powerful enough moves that are Super Effective. It helps if these moves receive STAB and any reduced damage is welcome.

One more thing to note is Ancient Power Rock has a 10% of increasing the user’s Attack and Defense 2 Stages. The below categories assume no Stat Boosts. If Altered Giratina gets even one Stat Boost, Arceus help us all.

Guaranteed Winners
These two counters will beat Altered Giratina in every shield count situation.
Wigglytuff	Feint Attack Dark	Play Rough Fairy Ice Beam Ice
Wigglytuff is the only true guaranteed winner. It can counter with either move set. Ice Beam Ice is better to use against Shields while Play Rough Fairy is a ringer when no Shields are left. Keep in mind that Wigglytuff is a slow counter.
Alola Ninetales	Powder Snow Ice	Ice Beam Ice
Alolan Ninetales can win in any situation against Altered Giratina; however, the battle is very close when you are at a Shield Disadvantage. Alolan Ninetales will counter a little quicker than Wigglytuff.
Will Win with Even Shield Counts
These counters will beat Altered Giratina when you and your opponent have the same Shield count.
Cloyster	Ice Shard Ice	Avalanche Ice
Mamoswine	Powder Snow Ice	Avalanche Ice
Granbull	Snarl Dark	Play Rough Fairy
Aggron	Dragon Tail Dragon	Stone Edge Rock
Steelix	Dragon Tail Dragon	Crunch Dark
Will Win with a 1 Shield Advantage
These counters will beat Altered Giratina when you have one more Shield than your opponent.
Tyranitar	Bite Dark	Crunch Dark
Alola Muk	Bite Dark	Acid Spray Poison
Zangoose	Shadow Claw Ghost	Night Slash Dark
Lapras	Frost Breath Ice	Ice Beam Ice

Giratina Origin PvP counters

Origin Giratina DragonGhost
Effective Stats at Level 25.5 (IV 0/13/14)
ATK 225		DEF 187		HP 284
Weak to			Strong against
Ice Ghost Fairy Dragon Dark			Bug Electric Fire Grass Poison Water Fighting Normal x2
Best PvP move set for Altered Giratina
Shadow Claw Ghost		Shadow Ball Ghost		Dragon Pulse Dragon
Super Effective versus
Psychic Ghost		Psychic Ghost		Dragon
Move Stats
Power 6 Energy 8 Duration 1 DPT 6 EPT 8		Power 100 Energy 55 DPE 1.82		Power 60 Energy 90 DPE 1.50 Dust cost 100000 Candy cost 100

Origin Giratina trades high Defense for high Attack while keeping its high Stamina Stat. This translates to using a similar type Pokémon as a counter – high Attack with reasonable bulk. Super Effective moves, STAB, and reduced damage are definitely pluses.

Guaranteed Winners
These counters will beat Origin Giratina in every shield count situation.
Skuntank	Bite Dark	Crunch Dark
Tyranitar	Bite Dark	Crunch Dark
Skuntank and Tyranitar perform nearly identical against Origin Giratina in Ultra League battles. Bite alone will dispense of Origin Giratina with incredible speed, putting these two on top if your opponent has any shields left.
Umbreon	Snarl Dark	Foul Play Dark
Will Win with a 1 Shield Disadvantage
Zangoose is the only counters that will beat Origin Giratina, in less time than Umbreon, and not listed above when your opponent has one more Shield than you do.
Zangoose	Shadow Claw Ghost	Night Slash Dark
Will Win with Even Shield Counts
These counters will beat Origin Giratina when you and your opponent have the same Shield count.
Drapion	Bite Dark	Crunch Dark
Alola Muk	Bite Dark	Dark Pulse Dark
Exploud	Bite Dark	Crunch Dark Disarming Voice Fairy
Both Crunch Dark and Disarming Voice Fairy have identical PvP Stats – Power 70 and Energy 45. Neither receives STAB, but both are Super Effective against Origin Giratina.
Sneasel	Ice Shard Ice	Avalanche Ice
Weavile	Ice Shard Ice	Avalanche Ice
Sneasel and Weavile perform nearly identical against Origin Giratina in Ultra League battles. Sneasel takes the advantage due to its lower resource cost.
Houndoom	Snarl Dark	Crunch Dark Foul Play Dark
Both Crunch Dark and Foul Play Dark have identical PvP Stats – Power 70 and Energy 45. Both receive STAB and are Super Effective against Origin Giratina.
Cloyster	Ice Shard Ice	Avalanche Ice
Piloswine	Powder Snow Ice	Avalanche Ice
Mamoswine	Powder Snow Ice	Avalanche Ice
Piloswine and Mamoswine perform nearly identical against Origin Giratina in Ultra League battles. Piloswine takes the advantage due to its lower resource cost.