Sorry for the delay, but I finally managed to get enough free time to get this done.
Terrakion @ Choice Scarf
Ability: Justified
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly / Naive Nature
- Close Combat
- Stone Edge
- Iron Head / Rock Slide
- Iron Head / Rock Slide / X-Scissor / Earthquake / Toxic / HP Ice
Role: Revenge Killer, Late-Game Cleaner
What It Does: While Terrakion may not be quite the monster he once was in OU, he's still a force to be reckoned with. Terrakion has an excellent physical attack stat of 129, giving him great power, and his 108 Speed allows him to pull off a scarf set fairly effectively. He also has a good bulk of 91 / 90 / 90, giving him the ability to take a few hits if necessary. Combined, these qualities make him an excellent, though by no means flawless, revenge killer. Terrakion is especially effective at cleaning up late game, once all of his checks are down. Close Combat is the mandatory powerful fighting STAB. Stone Edge provides a powerful Rock STAB. In slot 3, either Iron Head or Rock Slide can be run. Iron Head is very useful in this meta. The abundance of Fairies make it a good idea for Terrakion to run this, since only one Fairy (Azumarill) has any chance of going before Terrakion. Concealing the Choice Scarf by sending Terrakion out against something it would outspeed no matter what is a good way to lure Fairies like Mega Diancie in and take them out with Iron Head, as they might be expecting to outspeed Terrakion. Rock Slide, on the other hand, is arguably just as useful, since it provides a more reliable alternative to Stone Edge. Rock Slide makes cleaning significantly easier, as it retains a fair amount of power, has a nice flinch chance, and doesn't suffer from Stone Edge's infamous accuracy issues. Both moves are very useful for Terrakion, so which you choose comes down to a simple matter of what you think your team will benefit the most from Terrakion having a more reliable answer to. An alternative is to run both moves, negating the problem of choosing entirely, though this may deny Terrakion a useful coverage move. There are quite a few other moves that could be used in the final slot, depending on what you need Terrakion to revenge kill. X-Scissor gives Terrakion an option to hit
most psychic types hard. Given Terrakion's weakness to them, it is best used on predicted switches (especially against potential Scarf material like Latios), unless Terrakion is guaranteed to outspeed and OHKO. Earthquake is a highly damaging move that provides excellent coverage with Stone Edge, and gives Terrakion a good way to damage certain Steel types that are not hit super-effectively by Close Combat, such as Mega Metagross. One more option for the last slot would be Toxic. While this denies Terrakion a fourth coverage move, it does provide him with a better way of punishing bulky switch-ins he wouldn't otherwise be able to do much against. One final option for the final slot is Hidden Power Ice. While it may seem odd to run a Special Attack for coverage on a Pokemon with a mediocre SAtk of 72, HP Ice does allow Terrakion to lure in Garchomp and either form of Lando (and, to a much lesser extent, Gliscor) and hit them fairly hard on the switch, likely 2HKOing them. The EVs and nature are pretty standard.
Good Teammates: Due to Terrakion's speed tier, he can outspeed most common Dragon Dance users even after 1 Dragon Dance. For this reason, any team that has problems with Charizard X, Mega Altaria, Mega Gyarados, Dragonite, or Mega Tyranitar all appreciate having Terrakion around, as he (depending on what coverage moves are used) possesses super-effective attacks against all of them. Likewise, with Terrakion's speed, he is capable of outspeeding the vast majority of Pokemon that are covered by one of his attacks and taking them down pretty easily. Given just how many types he is capable of covering, this means Terrakion can fit on a great many teams with little difficulty. A specific example of a Pokemon benefiting from Terrakion's presence would be Mega Alakazam, as many of Mega Alakazam's checks and counters (such as Bisharp, Tyranitar, Talonflame, and Mega Lopunny) are handled fairly effectively by Terrakion. In terms of partners Terrakion likes having around, it is very important to have reliable hazard setters on your side, as hazards can help ensure critical OHKOs for Terrakion. Water Pokemon are a major problem for Terrakion, as he lacks any kind of coverage against them. Especially dangerous is Azumarill, since the Water typing nullifies any steel weakness, while CB boosted Aqua Jet destroys Terrakion with Priority. Thundurus, Celebi, Rotom-W, Venusaur, Raikou, Serperior, and many others are capable of filling this role. Terrakion also has some problems with physically bulky Ground types such as Hippowdon. Potential answers to them include Rotom-W, Venusaur, Celebi, Starmie, Slowbro, Serperior, and Gyarados are all capable of filling this role. Something to handle Scizor, who ruins Terrakion's day with Bullet Punch, is advised too. Almost anything capable of using a good fire attack, such as Talonflame, Heatran, Charizard X or Y, and many others are all great for this. Depending on what moves are run, it will be necessary to have Pokemon capable of taking on Psychic and Fairy types, as well as something with Ice attacks for Lando, Garchomp, and Gliscor.
What Counters It: Anything with priority that is super-effective against Terrakion, and that Terrakion can't reliably KO on the switch will always force Terrakion out. Both Scizor and Azumarill are excellent in this regard, as Terrakion can do very little against either of them on the switch. Conkeldurr is also worth mentioning, as Mach Punch also hits Terrakion super-effectively. While Terrakion can generally outspeed any other Fairy, it has to be very careful switching in on them. Anything with Will-O-Wisp can put a stop to Terrakion's run pretty quickly. As mentioned above, any bulky Water or Ground types are pretty safe switch-ins (unless Terrakion is running Toxic). Examples would include Suicune, Slowbro, and Hippowdon. Terrakion is also very vulnerable against Psychic types unless running X-Scissor to hit them on switches. Scarf Latios is especially dangerous as it completely outspeeds Terrakion. Mega Metagross also has very little to fear from Terrakion. If Terrakion is running Earthquake, he can hit MegaGross on the switch, but he will still be forced out. While Terrakion isn't really a case of 4MSS, he is going to end up with a major vulnerability to
something, depending on what coverage moves you're running. Don't run Iron Head, and Fairy types become a problem. Don't run X-Scissor, and Terrakion will always be forced out by Psychic types, with no guarantee of taking them out on the switch. Don't run HP Ice, and Lando, Garchomp, and Gliscor become problems. Don't run Rock Slide, and even the Pokemon Terrakion is supposed to check or counter may stand a chance against him due to Stone Edge's decidedly less than stellar accuracy.
Any Additional Info: While Terrakion does have a little bit of bulk, he's going to be worn down pretty quickly. Play conservatively with him early on, and save him for later on when the opponent's team is worn down. If you do need to bring him in earlier, try to bring him in on resisted hits or free switches. Don't let him get burned, or you will deeply regret it. Also, if you put Terrakion on a team with a member who can use Focus Blast, you may notice that Stone Edge seems to miss twice as often as Focus Blast despite being supposedly 10% more accurate. Ah, the mysteries of life.
EDIT: Okay, I made some significant changes based on the comments below. Hopefully, the set as I have listed it will be satisfactory now. For the record, I think the best set is probably CC, Stone Edge, Rock Slide, and Iron Head, but I'll be leaving the other options in the post as options for the fourth slot, as they are all usable. Also, before someone makes claims of being satisfied, I'd like to point out that what I have here is a compromise. The essence of compromise is that no party is completely happy with the result. If you vehemently believe that this needs to be changed further, you're more than welcome to take it up further with me, however.