Moves
U-turn lets Dialga pivot out to a more offensive Pokemon to gain momentum, which is good considering that Dialga is moderately slow. Core Enforcer is a 100-Base Power STAB attack that can negate the abilities of faster foes, which is especially handy considering that Dialga either underspeeds or Speed ties with many other Pokemon in the metagame, especially most Dragon-types bar Zygarde-C. Revelation Dance becomes Steel-type for Dialga and can hit Mega Gengar even after it changes Dialga's ability to Normalize. It also threatens Fairy-types like Mega Audino and Xerneas. Spectral Thief lets Dialga steal boosts from Pokemon utilizing setup moves to prevent Dialga from getting overpowered too quickly. It also hits Mega Mewtwo Y, Aegislash, and Mega Gengar, which Dialga checks decently well. Rapid Spin can be used to get rid of entry hazards, as Dialga is difficult to spinblock due to it threatening two of the three most common Ghost-types, namely Giratina and Mega Gengar. Nuzzle can be used to paralyze targets, but it makes Dialga more difficult to Imposter-proof.
Set Details
A Sassy nature lets Dialga maximize its Special Defense and minimize its Speed. That alongside 0 Speed IVs lets Dialga be as slow as possible to ensure Core Enforcer usually hits last, to prevent Dialga from getting its own ability disabled by other Core Enforcer users, and to let Dialga be a slow pivot. 16 Speed EVs with a Careful nature is an alternative option that allows Dialga to steal Shell Smash boosts from various Pokemon and outspeed everything up to Mega Mewtwo Y at +2 Speed, as it can survive attacks from quite a few Shell Smash users provided they don't have STAB Power Trip or super effective coverage against Dialga. Dialga trades the ability to use status moves for more Special Defense with its Assault Vest. With a 1.5x Special Defense, it can take on many special attackers in the tier such as Primal Kyogre, Mega Gengar, and Xerneas. Regenerator restores 33% of Dialga's HP every time it switches out, letting Dialga passively recover without wasting any turns. This works especially well with momentum moves like U-turn. Regenerator is an important ability on this set, as the passive healing makes up for Dialga's inability to use recovery moves because of its Assault Vest.
Usage Tips
RegenVest Dialga counters most special attackers that don't have super effective coverage against it, such as Primal Kyogre without Tinted Lens, Normalize Mega Gengar, No Guard Mega Mewtwo Y, and Xerneas lacking Earth Power. Its improved bulk allows it to switch into these threats and then heal over the course of the match by pivoting or switching out. If Dialga is at low health, utilize free turns against passive Pokemon by switching Dialga in then double switching or using U-turn. Always use U-turn liberally, as oftentimes, once your opponent knows your Dialga is defensive, they'll attempt to bring in walls because they know Dialga won't be able to do too much aside from use Core Enforcer. If they bring in a wall on the same turn Dialga pivots, you can bring in a wallbreaker and maintain momentum. If you predict your opponent is going to bring in something frail, you can try to get a 2HKO or 3HKO using a STAB move. Be wary of Knock Off, however, as Dialga gets significantly weaker if its Assault Vest is removed, so be careful around common Knock Off users such as Mega Audino and Mega Tyranitar even if Dialga has the type advantage. Don't waste Core Enforcer's mere 16 PP, as the move serves both an offensive and utility purpose. If you can't knock something out, pivot to something that can, as Dialga's main role is to provide momentum for the team through pivoting.
Team Options
Imposter Chansey can assist in scouting for sets that would threaten Dialga like Secret Sword Mega Mewtwo Y as well as various other threatening abilities like Adaptability. Dialga absolutely must have a teammate that can deal with Mega Mewtwo X, as Mega Mewtwo X can easily force out Dialga and set up if it predicts Dialga will switch. Mega Sableye, Giratina, and Mega Slowbro are all checks to Mega Mewtwo X. Zygarde-C can also beat most sets if it has Unaware or Prankster. It is also important to check Mega Diancie, as it is one of the few Fairy-types that can beat Dialga if it has Precipice Blades, and a Boomburst from Mega Diancie is a nearly guaranteed 2HKO with Stealth Rock damage. Ho-Oh beats the Pixilate variants of Mega Diancie, while Flash Fire Aegislash and Sturdy Shedinja beat Magic Guard variants. Red Orb Groudon can prevent Mega Diancie from becoming a problem, and it can also set entry hazards to beat one of Dialga's counters in Sturdy Shedinja. Red Orb Groudon also patches up any weaknesses against opposing Steel-types that Dialga faces and beats opposing RegenVest Pokemon with its powerful physical attacks. Besides Primal Groudon, Mega Mewtwo X can also deal with opposing Steel-types, as this variant of Dialga cannot threaten them at all.
RegenVest Dialga can fit on many team archetypes as it can passively recover health and support its teammates by pivoting, and can be paired with decently frail sweepers and wallbreakers like Mega Mewtwo Y. It also pairs well with teammates like Slaking, Mega Sableye, and Zygarde-C that are normally forced out by powerful special attackers. Dialga hates Spikes as they limit its pivoting ability and prevent it from walling stronger attacks like Mega Diancie's Boomburst and Tinted Lens Primal Kyogre's Steam Eruption. Thus, if Dialga doesn't run Rapid Spin, another teammate should run Rapid Spin or Defog. Dialga appreciates something that can absorb Knock Off, as its ability to wall is severely hampered if it loses its Assault Vest. Griseous Orb Giratina, Red Orb Groudon, and Pokemon with Z-Crystals cannot have their items knocked off. Unaware users such as Primal Kyogre, Mega Gyarados, and Mega Audino are also appreciated partners to Dialga as they can beat the setup sweepers that Dialga struggles with. This set is relatively easy to Imposter-proof, as many special walls like Primal Kyogre can easily switch into Imposter users transformed into Dialga, but a drawback is that Imposter users can come in and heal for free, which is why you may want something that can both switch in and force out Imposter Pokemon. A teammate with Poison Heal can help alleviate any issues with Nuzzle spreading status and Poison Heal also soaks other statuses that Dialga is weak to such as sleep from Spore. If you are not running Nuzzle, you can opt for a Safety Goggles teammate instead to check Spore users like Xerneas.