While using an NFE not named Chansey in OU might seem odd, especially one with such pitiful stats as Aron, Aron has quite a bit going for it. It succeeds where all other FEAR users have failed in the past, mainly due to its immunities to Toxic and sandstorm. This means that unlike Pokemon such as Cottonee and the classic Rattata, Aron cannot be worn down by most common forms of residual damage. It actually thrives in sand, as sandstorm damage finishes off most Pokemon after they take an Endeavor, mitigating Aron's lack of access to priority. After Aron is hit by an attack, which will always activate Sturdy because it is level 1, it can use Endeavor to bring the foe down to 1 HP. Aron's Shell Bell then heals it back up to full HP, creating an endless cycle of death.
Like most Pokemon, Aron does have its flaws. First off, most opponents know exactly what Aron does, so they can play around it. Aron's major downfall is its vulnerability to entry hazards; if they are on the field, Aron cannot switch in lest its beloved Sturdy be broken. Additionally, it is affected by burn, Leech Seed, Encore, and Perish Song, not to mention multi-hit moves, which ignore Sturdy. Furthermore, Aron flat-out fails against stall and even some balance teams, as they always carry at least one or two of the aforementioned ways of defeating Aron. However, offensive teams generally have a difficult time against it; these teams are the most common, so Aron finds use in most battles. Overall, despite its numerous downfalls, Aron is a decent choice for any team, though you must provide the support it requires.
Aron isn't that difficult to use, but there are a few steps you must take for it to succeed. During the Team Preview, look over your opponent's team. If he or she does not have a Ghost-type or a Pokemon that can inflict burn, then Aron has the upper hand. However, if the opponent does, these Pokemon and others that can stop Aron, such as users of Perish Song, Encore, and multi-hit attacks, must be removed. As the game progresses and these issues are dealt with, the next step is getting Aron in, which isn't that easy. If there are entry hazards on the field, make sure they are gone before you bring in Aron, as they will break its Sturdy and thus render it completely useless. Aron also has to come in completely unscathed, so you must be sure to utilize smart double switches, U-turn, and Volt Switch; the only other option is to bring it in after one of Aron's teammates faints. An interesting way to do this is to use a kamikaze Explosion and then bring it in.
Once Aron is in play, you are nearly guaranteed one kill. Any attack will bring Aron down to Sturdy range, so the best move is to respond with Endeavor, after which Shell Bell will bring it back to full health. If the foe doesn't attack, you have a few options. This is where the other moves come into play. If the foe doesn't carry Leftovers, Protect is an excellent choice, as sandstorm damage will finish these foes off, assuming they are damaged by it. If the foe isn't affected by sandstorm or is a Ghost-type, using Toxic is the best modus operandi, as this will wear said foe down. Although Aron itself might not be able to complete the job, a teammate should be able to. Hidden Power Rock is for cases where the foe is immune to sandstorm damage and Toxic yet is sitting at 1 HP. Hidden Power Rock is chosen because every attack will do exactly two damage to a level 100 Pokemon regardless of its Base Power, and it can hit Shedinja. If the foe is immune to Toxic and sandstorm and carries Leftovers, Aron just will not be able to kill it, though it will come very close. Sometimes, however, you don't have to deal with any of this due to the foe being unprepared for Aron and can just spam Endeavor along with possibly Protect to win the game.Team Options & Additional Comments >>>
Using any set other than the one listed will cause you to be the laughingstock of the OU ladder. Don't even think about it.
Checks and Counters
As a start, any weather other than sand renders Aron completely useless. Without the additional damage sandstorm provides, Aron is unable to actually kill its foes. Ghost-types, such as Jellicent, Gengar, and Sableye, are unaffected by Endeavor and thus are perfect counters to it. All Aron can do in return is use Toxic, though Gengar is immune to it too. Jellicent and Sableye in addition to Rotom-W can burn Aron; because Aron is not immune to burn, it is immediately crippled for the duration of the match. Because Aron is affected by Leech Seed, Pokemon such as Ferrothorn can also stop Aron. Entry hazards also render Aron completely useless, as they break its beloved Sturdy, which means it can't take a hit before it uses Endeavor.
Defensive Pokemon that are immune to Toxic and have some sort of recovery option, such as Ferrothorn and Skarmory, though Ferrothorn won't enjoy a hit from Endeavor. Poison Heal Gliscor is especially effective, as its ability heals it up after the Endeavor; it is also immune to Toxic and sandstorm. Pokemon that carry multi-hit attacks, such as Breloom and Cloyster, can ignore Sturdy and kill Aron. Pokemon with the ability Mold Breaker or Teravolt work in a similar vein. Garchomp and Ferrothorn, which have the abilities Rough Skin and Iron Barbs respectively, can check Aron; their abilities can finish off Aron after it uses Endeavor, though the former pretty much has to sacrifice itself to do so,. Finally, having unrelenting offensive pressure means Aron cannot switch in, so if you somehow lack any of the aforementioned Pokemon, playing well can get you around it.