[Overview]
Registeel sports fantastic bulk, and when compared to Cresselia, it's bulk is equal, if not better. It also sports a fantastic steel typing which makes it easy to switch into. Registeel can take multiple hits, and has access to a recovery move in the form of rest. It is great for changing weather conditions, through the moves Sunny Day, and Rain Dance, which upsets every weather team. Then it has access to the crippling Thunder Wave, which is always great since it's typing, and bulk allows it to set up without much worry. It's Steel typing has it's disadvantages too. Leaving it weak to three common types (Fire, Fighting, Ground), which do not share a common Physical, or Special liking. And if two Pokemon go for Registeel, with strong moves, it's done for. But, there is a move which will make Registeel a super-fast mon, and will allow it to become amazing support.
[SET]
Name: Anti-Weather Trick Room
Move 1: Swagger/Rain Dance/Sunny Day
Move 2: Charge Beam
Move 3: Sunny Day/Rain Dance
Move 4: Protect/Rest
Item: Leftovers/Chesto Berry
Nature: Relaxed/Sassy
Ev's: 252 HP/ 124 Def/ 8 SpAtk/ 124 SpDef
[SET COMMENTS]
This set is possibly the greatest anti-weather set that Registeel can possibly utilize. To start off the first move has several options. You can use the popular worlds strategy of Swagger, or Rain Dance or Sunny Day which can be used for maximum anti-weather coverage, just since Rain Dance ruins everything except rain, and Sunny Day ruins everything except sun. Charge Beam is a great move that has a great chance to boost Registeel's average Special Attack, and can also be used to counter that fat bulky Politoed,or additional water types. Then the third move slot will be anti-weather support. I recommend Sunny Day over Rain Dance though, because Rain teams are a little bit more common. Then the last move also determines what it's gonna hold. Protect is useful to predict, scout, or take out a threat, while working in conjunction with Leftovers to restore a little HP. Or you could go for max HP restoration, and an immediate wake up call with the combo of Rest, and Chesto Berry.
[ADDITIONAL COMMENTS]
For this set, it would be better if Trick Room could go up so Registeel can have the upper-hand when it comes to who moves first. Registeel is not an attacker, and therefore set is susceptible to Taunt which it absolutely loathes. So Mental Herb is also an option to scout for Taunt. Registeel is probably the best anti-weather counter to Rain Teams, because they usually don't have mons that have super-effective moves against Registeel. Registeel can also be effective against Sun, to because of Rain Dance's hindrance.
[Other Options]
Registeel has access to some great support moves. You could give it Sleep Talk in place of it's first move, so it can work in conjunction with Rest. Thunder Wave can be in place of move umber 1, but it is counter productive on a Trick Room Registeel, but the paralysis is always useful. You can also replace it's first move with Curse, and it's second move with Iron Head, which works especially good with Trick Room since it has a chance to inflict a flinch. Then you could make it commit suicide, and give it a normal gem Explosion, which will deal massive damage. For the past two options you should alter it's Ev's for 8 into Attack, instead of Special Attack.
[Checks and Counters]
If Registeel is on the back of a super-effective move, with high base power and STAB, such as a Hi Jump Kick from Mienshao, it's chance of survival is decent, but it will be left with low HP. A gem Close Combat from Hitmontop also has a high chance of a OHKO, and same with a Guts boosted Close Combat from Hariyama. In addition, if a two super-effective attacks hit Registeel for the same turn it's chances of survival are fairly low. So ground, fire, and fighting types do the best at countering Registeel.
