Thundurus
Thundurus was such a powerful threat back in its short stint of OU, that it was banned. This is due to its excellent dual 115/125 base offensive stats along with its high 111 base Speed, immunity to Spikes and Toxic Spikes, Prankster ability, and Nasty Plot. These traits allow it to fulfill two roles: team supporter and sweeper. The team support set functions mainly to cripple fast sweepers in an emergency by paralyzing them, so that you can easily revenge them. The sweeper set wields the most powerful Thunder in the game after Nasty Plot, and is quite hard to stop once it has set up. Thundurus has a fair amount of problems though. Its 79/70/80 make it among the frailest Pokemon. It is also cursed with a weakness to Stealth Rock. Thundurus is still a huge threat, so don't go overlooking it.
Abilities
Prankster: Very useful ability that gives +1 priority to a non-attacking move. It isn't very useful on sweeping sets though, as most Pokemon will just attack Thundurus rather than attempting to stop it from setting up by Taunting it.
Defiant: Gives +2 attack for every stat drop. By no means a bad ability, but is wasted on Thundurus.
Nasty Plot
Ability: Prankster
Item: Life Orb
Nature: Timid
EVs: 4 Def / 252 SpA / 252 Spe
- Thunder
- Hidden Power Ice
- Focus Blast
- Nasty Plot
This is probably the most common Thundurus you'll see, thanks to its sheer power after a Nasty Plot. Thunder is preferred as the main STAB move due to rain being prominent in ubers, its high power, and 30% paralysis chance. Hidden Power Ice is there to hit the Grass- and Ground- types that try to Switch into Thunder. Focus Blast is in the last slot to hit Dialga, Kyurem's formes, and most notably, Ferrothorn.
Prankster
Ability: Prankster
Item: Life Orb / Focus Sash
Nature: Naive / Timid
EVs: 4 Atk / 252 SpA / 252 Spe.
- Thunder
- Thunder Wave
- Taunt / Hidden Power Ice
- Superpower / Volt Switch / Taunt
BW2 improved this sets effectiveness by giving it access to Superpower, which allows Thundurus to stay in a bit longer while still having the ability to hit Chansey and Blissey. Thunder is still very powerful even with no way of boosting. Thunder Wave is an important part of this set: it allows Thundurus to neuter fast sweeper and revenge killers so you will basically have a free chance to remove a important Pokemon on your opponent's team. Be aware, however, that this usually means Thundurus will die as well. Taunt is also a move that is effected by prankster, so stuff like Ferrothorn and Dialga can't set up on you.
Other Options
Thundurus has a few other options, but they aren't too useful. Grass Knot is most notable, as it has a chance of OHKOing max HP Groudon where Hidden Power Ice only 2HKOs, but the chance of OHKOing is small, and has no other use. Agility can be used to beat Choice Scarf users, but some of them will till beat Thundurus regardless.
Counters
The closest thing to a counter for it is Chansey, but it is 2HKOd by a +2 Focus Blast. Groudon can switch into Thunder, take a Hidden Power Ice, although it will do about 50%, and KO with Stone Edge. Like most sweepers, there are also a lot of revenge killers out there that destroy it. Mewtwo, Choice Scarf Palkia, and Terrakion, to name a few.