Moves
Heavy Slam gives Mega Aggron the ability to OHKO three huge threats to stall teams in Tapu Lele and Mega Diancie, as well as OHKO Clefable with little chip damage and 2HKO Tornadus-T and Mega Latios. Earthquake aims to hit Heatran, Mega Mawile, and Toxapex, relieving the pressure that all of them put on the rest of the team. It also can be used to hit Magnezone, in the case that Mega Aggron gets trapped by it. Ice Punch 2HKOes offensive Landorus-T, Garchomp, and Gliscor, which all take minimal damage from Mega Aggron's Heavy Slam, threaten Mega Aggron with super effective Ground-type STAB moves, and can pose a threat to stall teams depending on the set. Fire Punch has the same objective but instead targets Kartana, Mega Scizor, and Ferrothorn, achieving a 2HKO on all of them while having a favorable chance to OHKO the former. Protect helps Mega Aggron scout moves and waste either PP from the opposing team or turns from weather or screens. It can also serve as a way to make Mega Medicham lose half of its health by missing High Jump Kick, making it much easier to deal with. Toxic puts Tangrowth, Zapdos, and Rotom-W on a timer, since they aren't bothered by Mega Aggron's other moves.
Set Details
248 HP EVs, 200 Special Defense EVs, and a Careful nature allow Mega Aggron to endure two Psychics from Choice Specs Tapu Lele under Psychic Terrain without Stealth Rock damage, while 56 Attack EVs let it OHKO maximum HP Heatran with Earthquake almost every time. The remaining EVs are put into Speed to let Mega Aggron outspeed uninvested base 50s, such as Chansey and Alolan Muk. An offensive spread of 252 HP / 252 Atk / 4 SpD and an Adamant nature can be used in order to OHKO Garchomp and Gliscor with Ice Punch after Stealth Rock damage and deal more damage with Heavy Slam against the likes of Tornadus-T and Reuniclus, but the lack of special bulk investment can be detrimental in some situations.
Usage Tips
Try to lead with Aggron, especially if the last move that you are using is Protect, and Mega Evolve as soon as possible in order to get rid of its poor defensive typing. If it is not possible to Mega Evolve Aggron on the first turn, try to bring Aggron in with a free switch, as even a resisted special move can make Aggron lose a lot of its health and consequently make it more dependent on Wish to heal. Avoid letting Mega Aggron's HP get too low; if that occurs, Mega Aggron can become unable to pivot into the Pokemon that it normally could. When pivoting, try to bring it in against special attackers that fear and can't do much against it such as Tornadus-T without Focus Blast and Mega Latios or against physical attackers such as Mega Pinsir and Weavile. Entry hazards, specially Spikes, can be somewhat detrimental to Mega Aggon, as it turns some 3HKOs against it turn into 2HKOs, such as Offensive Landorus-T's Earthquake and Choice Specs Tapu Lele's Psychic, leaving it unable to pivot into those threats. Therefore, try to keep them off your side of the field in order to preserve Mega Aggron's ability to consistently swallow hits. Mega Aggron's ability to pressure opposing hazard setters, such as Clefable and Mega Diancie, must be used as much as possible, so try to predict the opponent switching one of them in and double switch into them; since Mega Aggron can OHKO the former with little chip damage and outright eliminate the latter with ease, they will be discouraged from attempting to set up hazards. However, beware of Mega Diancie's Earth Power, as it can weaken Mega Aggron to a point where it can no longer switch in on threats without the risk of being KOed. Also, be wary of switching in on coverage moves from Mega Mawile and Tapu Lele; for example, pivoting in on Focus Punch Mawile will make Mega Aggron lose more than half of its health, while pivoting in on Focus Blast Tapu Lele will result in Mega Aggron getting 2HKOed, and in both cases Mega Aggron will be unable to check the threats it used to. With this, scouting for those moves is fundamental.
Team Options
As Mega Aggron lacks a reliable form of recovery, Wish support is almost mandatory on teams with it. Chansey, Clefable, and Jirachi are all available and able to fit well on stall teams, with Chansey being the most effective, since its Wish heals all of Mega Aggron's HP, while Clefable forms a good Steel / Fairy core with it. Also, Chansey is a formidable counter to most special wallbreakers and sweepers, including Mega Charizard Y and Volcarona, beating them with ease for Mega Aggron. Entry hazard removers, specifically Zapdos and Mew, are good partners, as both check or counter some of Mega Aggron's checks, such as Celesteela in the case of Zapdos and Mega Medicham in Mew's situation, while keeping the field clear, making Mega Aggron's job of checking Pokemon easier. Since you're forgoing using Mega Sableye in the team's Mega slot, this kind of support is more indispensable than normal. Cleric support, or even a status absorber, is crucial to Mega Aggron teams. Chansey and Clefable can provide Heal Bell support to heal status, while Gliscor can pivot into status inducers, preventing other targets. Lastly, Buzzwole is an excellent partner to Mega Aggron, since it can pivot into the physical attackers that still can pose a threat to Mega Aggron, such as Tapu Bulu, Garchomp, Gliscor, and Mega Heracross.