Moves
Amnesia is one of Registeel's two defensive boosting moves, boosting its Special Defense by two stages in order to avoid KOs from special attackers like Genesect, Serperior, Timid Naganadel, and Choice Specs Necrozma. Iron Defense is Registeel's other boosting move, allowing it to withstand physical attacks from the likes of non-Choice Band Dragonite, Mega Metagross, and Choice Scarf Haxorus. Rest is Registeel's sole recovery move, allowing it to heal off damage before being KOed, with the sleep turns also aiding it in PP stalling opposing stall Pokemon like Jumpluff, Type: Null, and Kyurem. Seismic Toss is Registeel's best damaging move, dealing damage based on level, rather than its underwhelming offensive stats. Counter allows Registeel to heavily damage physical attackers that might otherwise overwhelm it, like Choice Band Dragonite, Choice Band Zeraora, Mega Tyranitar, and Mega Pinsir.
Set Details
172 EVs in Defense coupled with 248 HP allow Registeel to avoid the 3HKO from Dragonite's +4 Earthquake after boosting to +6 Defense. 88 EVs in Special Defense alongside the HP investment ensure that Registeel isn't KOed by Timid Naganadel's Fire Blast after using Amnesia. Leftovers effectively acts as extra bulk that allows the previous two points to be possible, in addition to preventing Registeel from losing to being locked into Encore by Primarina and Togekiss. Clear Body prevents Registeel's stats from being lowered, which is useful for blocking Metal Sound from the likes of Aegislash, Magnezone, and Celesteela.
Usage Tips
An important thing to note in PP stall matchups against Magearna, Mega Slowbro, Type: Null, and Mega Scizor is that you want Registeel to use Rest whenever it's at less than full HP so that you can save as much as possible, since Rest doesn't put Registeel to sleep otherwise. Against Rest Mega Slowbro and Type: Null, in particular, you need to play carefully, because Registeel actually ties them on PP. After both sides use up all of their boosting PP, Registeel wins by repeatedly using Rest, because it would either gain extra turns from sleeping or eventually KO them with Seismic Toss after they use up all of their Rest PP. This strategy can be a bit reliant on RNG against Mega Slowbro, as Registeel would be hoping for a Scald burn or critical hit so that it can use Rest at least six times. Against Choice Scarf Porygon-Z and Togekiss, it is ideal to use Seismic Toss on the first turn, so that you can KO them in the event that they try to lock Registeel into its first move with Trick. Even if Porygon-Z turns out to be a Z-Conversion set, Registeel can withstand just about any attack and boost up its Special Defense with Amnesia to stall it down. Against Primarina, it is ideal to use Amnesia on the first turn, even if you expect it to run Encore, as Registeel will eventually boost its Special Defense so much that Primarina's attacks won't be able to outdamage Leftovers recovery. Against non-Choice Band Zeraora, it is ideal to lead with Iron Defense, as a majority of them use physically oriented sets. However, if Zeraora uses Charge, it may potentially be a mixed set with Electrium Z and Thunder, meaning you should use Amnesia on the next turn. While the boosted Gigavolt Havoc will deal a lot of damage, Registeel is just bulky enough to withstand the following attack and can safely use Rest and eventually get to a comfortable position to KO with Seismic Toss.
Team Options
Registeel pairs well with Water-types like Mega Gyarados, Greninja, Mega Slowbro, Primarina, and Tapu Fini, as these Pokemon cover Registeel's glaring weaknesses to the Fire- and Ground-types that occupy the metagame. In return, Registeel covers their weaknesses to Grass- and Electric-types like non-Choice Band Zeraora, Magnezone, Mega Venusaur, Serperior, and Ferrothorn. Registeel also appreciates partners like Dragonite, Magearna, Tapu Lele, and Clefable that cover its weakness to Fighting-types. In return, Registeel can go head to head with many of the Steel-types like opposing Magearna, Mega Metagross, non-Laser Focus Aegislash, and Celesteela that would otherwise threaten these partners. While Registeel is considerably bulky, it still struggles with the plethora of Pokemon that use Earthquake, so it likes to have physically bulky teammates like Mega Slowbro, Zygarde, Mega Altaria, and Mega Sableye that can deal with them. In turn, Registeel can handle the special attackers like Choice Specs Greninja, Porygon-Z, Mega Gardevoir, Primarina, and Serperior that would otherwise overwhelm them.