In Pokemon Scarlet & Violet, making use of Terastallization is the key to succeeding in Pokemon battles. The new mechanic, introduced in the ninth-generation games, allows players to transform their Pokemon into any of the 18 types introduced throughout the franchise. Their Pokemon get each type's defensive resistances, as well as offensive STAB bonuses.
The best way to acquire Pokemon with specific Tera types is by completing Tera Raids, which allows players to link up with others online in a 4v1 battle. These fights can often test players' strength, patience, and strategy as they try to catch any of the available Pokemon. However, these six Pokemon Tera Raid fights have made players lose their minds the most.
6 Corviknight
It makes sense that Tera Raids would be especially hard against Pokemon who are already powerful in a generation's meta game. That's true of Corviknight, the Flying/Steel-type bird Pokemon introduced in the 8th generation games, Pokemon Sword & Shield. However, the Pokemon really shines in Scarlet & Violet as a threat to Tera Raid participants.
Particularly, Corviknight is a serious threat to Fairy-types, meaning players will have to forego bringing those Pokemon into battle. The best bet would be a Pokemon like Gholdengo, who can naturally resist any of Corviknight's moves, although acquiring a Gholdengo is not easy, considering you must first find 999 Gimmighoul coins scattered across the Paldea region.
5 Garchomp
Pseudo-legendary Pokemon are often much stronger than regular Pokemon, but not quite as powerful or rare as legendaries. As such, these Pokemon can be quite difficult to face during Tera Raids, especially when they have diverse enough movepools. That's true of Garchomp, a Dragon/Ground-type Pokemon introduced in the fourth-generation games.
Given Garchomp's access to powerful moves like Dragon Rush and Take Down, it would be unwise to challenge its Tera Raid without being prepared. This means bringing on a Fairy-type Pokemon, though the best choice of Ice/Fairy-type Alolan Ninetales isn't available to catch in Scarlet & Violet, at least before the release of the DLC expansions.
4 Annihilape
Annihilape is one of the strongest Pokemon introduced in the 9th generation. Evolving from Primeape, Annihilape boasts a strong typing of Fighting/Ghost, as well as access to the move Rage Fist, which increases power by 50 every time the Pokemon is hit. This is especially devastating during Tera Raids, where targets will take an enormous amount of hits.
The only edge players will have against this move would be to bring a Normal-type Pokemon, as they're immune to Rage Fist. It'd be especially useful to bring along a Hisuian Zoroark, given its unique Normal/Ghost-typing, which allows for STAB moves against Annihilape. However, even types resistant to Ghost will eventually be worn down by Rage Fist.
3 Dondozo
The metagame of Scarlet & Violet has really been shaken up by the presence of Dondozo, a new Water-type Pokemon. When used in Double Battles alongside the Water/Dragon-type Tatsugiri, its signature move Order Up will interact with Tatsugiri's ability and turn them into one extremely powerful unit that can be hard to beat, but this is not possible via Tera Raids.
Nevertheless, Dondozo still poses a real threat in its normal form due to its massive HP stat, which can make it hard to kill over time. Add in the fact that it could be disguised as a Tera type with even fewer weaknesses than Water-type, and Dondozo makes for an incredibly difficult fight for any group of four trainers.
2 Delphox
Delphox is the fully-evolved form of Fennekin, the Fire-type starter Pokemon from Pokemon X & Pokemon Y. It was first made available in Scarlet & Violet via a special event 7-star Tera Raid under the Fairy-type, which posed quite a problem for Fairy's enemy Steel-types, given Delphox's access to Fire-type moves and ability to Will-o-Wisp opposing Pokemon.
While some players may get lucky by porting over Pokemon like Heatran and Zacian through Pokemon Home, other players should probably look towards a good build of the version-exclusive ninth-generation Pokemon, Ceruledge and Armarouge. However, they'll still struggle to battle Delphox thanks to its access to Magic Room, preventing held item usage.
1 Greninja
By far the most infamously difficult Tera Raid in Pokemon Scarlet & Violet was Delphox's fellow sixth-generation fully-evolved starter, the Water/Dark-type Greninja. Facing a 7-star Poison-type Greninja was quite tough for many players, especially considering Greninja's popularity among Pokemon fans due to its anime appearances as a Super Smash Bros. Ultimate character.
This Greninja Tera Raid certainly earned the name "Greninja the Unrivaled" due to its sheer difficulty to defeat. The best bet for players was using a good build of Clodsire, given its Poison/Ground-typing and access to the ability Water Absorb, making it immune to Greninja's Water-type and Poison-type attacks, though Clodsire is still not a great offensive choice.