Aggron is a neat Pokémon with good stat distribution and a good movepool—it is just hampered by two terrible 4x weaknesses to very common attack types. Aggron is a real force in the UU environment, battering everything in sight with STAB Head Smash from an impressive base 110 Attack.
What sets Aggron apart from other Rock-types is its Steel typing. This gives Aggron a whole slew of resistances, which means it will have an easier time switching into moves such as Ice Beam and U-turn than most Rock-types.
With Choice Band and a Jolly nature, Aggron reaches an astounding 478 Attack. Aggron is easily capable of switching into battle thanks to its numerous resistances. Head Smash is Aggron's main form of attack due to its absurd Base Power, good offensive typing, and STAB. Aqua Tail pairs nicely with Head Smash, hitting those Pokemon hoping to switch into a Rock attack (such as Rock/Ground-types) for super effective damage. Low Kick nails Pokemon such as Steelix, Regirock, and Registeel for an easy 2HKO, while Ice Punch destroys Torterra. If you don't fear Torterra setting up, and you need a way to reliably take out bulky Grass or Water Pokemon, Toxic is a good option over Ice Punch.
With a Jolly nature, Aggron will almost always outrun defensive Milotic, and basically any other defensive Pokemon. However, an Adamant nature is viable, boosting Aggron's Attack to 525, meaning Aggron will hit that much harder. Aggron still outpaces most of the walls in UU, but there are defensive Pokemon, such as Milotic and Venusaur, who can easily be faster than Aggron with just 8 or 12 EVs invested in Speed.Team Options & Additional Comments >>>
Aggron has decent bulk with some HP investment, especially on the physical side, allowing it to easily set up a Substitute on many Pokemon. It also helps that Aggron has quite the ability to force switches with its dangerous 150 Base Power Head Smash attack. The major thing holding Aggron back is its 4x weakness to the ever-common Ground-type attacks, so that's where Magnet Rise comes in. With Magnet Rise, Aggron is barely touchable by its usual counters, who rely on Earthquake to hit Aggron. This means Pokemon such as Donphan will be forced to use Stone Edge, which merely 3HKOes Aggron's Substitute. Pokemon such as Steelix and Donphan are also hit hard by Focus Punch, another 150 Base Power move (though it lacks STAB). The strategy is pretty straightforward: you come in on one of the many attacks that Aggron resists and set up a Substitute. If the opponent switches in a Pokemon such as the previously mentioned Donphan or Steelix, use Magnet Rise. Proceed to destroy your opponent's team from behind your Substitute with Head Smash and Focus Punch. If your opponent has Roar on their Steelix or Donphan, it may be wise to just use Focus Punch instead of Magnet Rise, especially if you have used Magnet Rise earlier in the match.
84 Speed outruns minimum Speed Clefable and Weezing, allowing Aggron to hit them with Head Smash or set up Substitute before getting hit by a status effect. This also helps against Brave Cloyster who runs Surf and would otherwise break your Substitute or even OHKO you before you can OHKO it. 160 HP EVs reaches a Leftovers point +1 HP, allowing Aggron to use Substitute five times from full health. The remaining EVs are put in Special Defense to allow Aggron to more easily switch into Choiced Shadow Ball and Dark Pulse, but they can also be placed into Speed.Team Options & Additional Comments >>>
With Head Smash and a very respectable Attack stat, the only thing stopping Aggron from plowing through teams is its poor Speed. Of course, this can be remedied by Rock Polish, which boosts Aggron's Speed to a whopping 398, outrunning Pokemon such as Dugtrio, Alakazam, and Sceptile, even with a Speed boosting nature. It also outpaces neutral natured base 80s with Choice Scarf, but with a Jolly nature, Aggron is faster than base 80s with Choice Scarf and a Speed boosting nature; however, it really appreciates the power from Adamant, as it secures 2HKOs on Tangrowth and Regirock, while giving Aggron a much higher chance of OHKOing Rhyperior.
Aggron needs to focus on super effective coverage, as Head Smash is so powerful that it will do more than almost any other attack even when resisted. Aqua Tail 2HKOes Donphan and OHKOes almost every Rock- or Ground-type, which are Aggron's most common switch-ins. Low Kick 2HKOes Regirock and can KO Registeel if it switches into Stealth Rock and a Head Smash. Ice Punch can be used over Low Kick if you prefer beating Torterra, who is KOed after switching into Head Smash.Team Options & Additional Comments >>>
Aggron can run Screech in order to beat some of its common counters. After a Screech, Head Smash OHKOes most Pokemon without a Rock resistance, and Donphan, Claydol, and Rhyperior are all KOed by Aqua Tail. Since it doesn't need an Adamant nature to attain these KOes, Aggron can run a Jolly nature, which allows it to outpace defensive Milotic and OHKO it with Head Smash after Screech. However, most of Aggron's common switch-ins are 2HKOed by Head Smash or by a Head Smash followed by the appropriate coverage move anyway. As such, you are better off just outright attacking in most cases.
Aggron could run a more defensive set with sandstorm support, but it is almost always outclassed by other bulky Rock-types due to its low HP. Stealth Rock is also an option as a filler, but Aggron will almost always be doing more harm by crushing the opponent with Head Smash.
Iron Tail is an option as well, especially if you pair Aggron with Gravity to compensate for its poor accuracy. It has decent coverage with Head Smash and has a chance to lower the target's Defense.
A decent Trick Room set could be used since Aggron is slow and hits like a mack truck; Head Smash will destroy almost anything that doesn't resist it. The trouble is when Trick Room isn't up, Aggron is too specially frail to do anything without getting OHKOed first. Even with Aggron's horrible Speed, Choice Scarf is a viable option. With a Choice Scarf, Aggron can outrun Pokemon with base 95 Speed, and it still hits hard enough to 2HKO Milotic after Stealth Rock.
Checks and Counters
Switching into a 225 Base Power (factoring in STAB) attack, from an Attack stat of 350, is no easy task. Resisting Head Smash isn't enough; you need to resist it and have impressive Defense. Donphan, Nidoqueen, and Claydol wall Aggron's Fighting and Rock attacks, but they need to beware of Choice Band Ice Punch and Aqua Tail. Milotic can switch in and OHKO Aggron if it packs 8 Speed EVs to outrun Aggron as well. Hitmontop can switch into Head Smash after an Intimidate and OHKO with a STAB Fighting attack. Quagsire and Gastrodon are only 3HKOed by Aggron's Head Smash as long as there are no Spikes on the field, and they can Recover and set up Curse. In OU, Swampert and Hippowdon are the two best counters since they resist Head Smash and aren't hit too hard by Aggron's coverage moves. They can both send STAB Earthquakes at Aggron, which will easily OHKO.
Revenge killing is far easier, especially if Rock Polish isn't in effect. Any Pokémon with a super effective special attack, or simply any Fighting- or Ground-type attack, will easily OHKO Aggron. Vacuum Wave works great if you need to revenge kill Aggron after Rock Polish, since it will likely not be able to survive it as long as it's coming from a decent Special Attack stat. Mach Punch will do well too, but Aggron can survive as long as it has a decent amount of HP left.
Most neutral special attacks will do good damage; just beware of Aggron's high attack power when using frail special sweepers.