Moves
Thunder Punch is a necessity for many of the bulky Water-types that do not take enough damage from Mega Metagross's Steel-type STAB attacks, particularly Mega Gyarados, non-Substitute Primarina, Tapu Fini, and Mega Blastoise. Earthquake is an essential move for combating opposing Steel-types, mainly Magnezone and rival Mega Metagross. Iron Head, while not as powerful as the popular Meteor Mash, has 100% accuracy, improving consistency against your opponents by not risking a miss. Additionally, Iron Head has a much more beneficial side effect in its 30% flinch chance, granting Mega Metagross opportunities to get a free turn of damage whenever it uses the move; this is particularly helpful for Pokemon where you really need a single spare turn to win, such as Heatran, Dragonite if you lack Ice Punch, and Magearna if you lack Substitute. Zen Headbutt can be used either as an alternative STAB move for hitting foes neutrally or as a coverage move. Like Iron Head, it also has a flinch chance, though it's only 20% as opposed to Iron Head's 30%, so it's not really something that can be relied on as effectively for flinches. The main threats that it provides coverage for are Kommo-o and Mega Venusaur, and it is more reliable for OHKOing Naganadel. Substitute is a helpful utility move for dancing around status move users, as well as avoiding significant damage from Counter, Metal Burst, and Z-Moves. It is most useful for Magearna, Aegislash, Mega Venusaur, Celesteela, and Mega Mawile. Ice Punch is mainly just a coverage move for the threatening Dragon- and Ground-type Pokemon that occupy the metagame, namely Dragonite, Mega Altaria, and Landorus-T, although Mega Metagross will still lose to Groundium Z Landorus-T sets. Bullet Punch is a STAB priority move that helps for chipping down opposing Pokemon or following up for a KO. Bullet Punch will benefit you the most against Magnezone, Choice Specs Greninja, Mega Mawile, and Naganadel if you lack Zen Headbutt. Rock Slide can be run alongside a speedy EV spread for the purpose of catching Mega Charizard Y and Volcarona by surprise. If you're lucky, you could also get a flinch against Mega Charizard X for a 2HKO. Meteor Mash can be run on a speedy EV spread as well, 2HKOing Choice Specs Necrozma, and putting Pheromosa and Z-Conversion Porygon-Z into Bullet Punch range.
Set Details
The 248 HP and 28 Defense EVs with an Impish Nature make Mega Metagross capable of withstanding a maximum Attack Mega Gyarados's Dragon Dance-boosted Crunch. The 64 Special Defense EVs also just barely enable Mega Metagross to withstand a Timid Choice Specs Naganadel's Fire Blast, among other significant special hits like Choice Specs Greninja's Dark Pulse, Magnezone's Electrium Z-boosted Zap Cannon, and Heatran's -2 Overheat after using Substitute. The 72 Speed EVs just barely creep past maximum Speed Smeargle in order to avoid being put to sleep, as well as outspeeding Adamant Mega Gyarados in order to avoid a potential 2HKO from Crunch. The 96 Attack EVs make Thunder Punch a near-guaranteed 2HKO against maximum HP Mega Gyarados, as well as maximizing Mega Metagross's general damage output without sacrificing any essential investments. Clear Body is an essential ability before Metagross Mega Evolves because it prevents Intimidate and moves like Charm from lowering Metagross's Attack, as well as stopping moves like Electroweb, Rock Tomb, and Bulldoze from lowering its Speed.
Mega Metagross can run maximum Attack and maximum Speed to outspeed Pokemon like Garchomp and Landorus-T and OHKO them with Ice Punch or become immune to their Ground moves with Magnet Rise. This is mandatory for both Rock Slide and Meteor Mash, as well. Outspeeding Porygon-Z also helps so that you can 2HKO it with the combination of Meteor Mash, before it boosts with Z-Conversion, and Bullet Punch. This, however, sacrifices many matchups against bulky offensive or faster Pokemon due to the bulk you sacrifice in order to do so, meaning you now lose to Mega Gyarados, Greninja, Magnezone, and Naganadel.
Usage Tips
Mega Metagross finds itself best suited against opposing bulky offensive threats like Dragonite, Mega Gyarados, Meloetta, Mega Tyranitar, and Mega Mawile. In most scenarios against offensive Pokemon like Mega Gyarados, Dragonite, and Greninja, Mega Metagross wants to deal as much damage as possible as fast as possible because these Pokemon often don't use moves that it would be beneficial to use Substitute against. In scenarios against certain Pokemon that rely on a single powerful attack or aim to hinder Mega Metagross with status moves, such as Primarina and stall Celesteela, using Substitute immediately will help you be able to negate these moves so that you can go for the KO unhindered; however, some Pokemon possess the means to break its Substitutes, which means some predictions may be required, namely against foes like Aegislash and Hidden Power Fire Mega Venusaur. Against Magearna, you're best suited using Earthquake Turn 1 in order to get as much damage off as possible before it uses Iron Defense, as well as 2HKO it in the event that it's a Choice Specs set, then following up with Substitute so that it can't recover its HP by using Pain Split. Even though Greninja is a Water-type, it loses that typing upon using Dark Pulse, which means that you're better off using Mega Metagross's Steel STAB attack, rather than Thunder Punch; even if they use Hydro Cannon to resist Steel, they'll be stuck recharging the next turn, making them easy to pick off with Thunder Punch. Be forewarned that you will need Bullet Punch in order to reliably 2HKO Greninja if it uses Dark Pulse, as Iron Head does not have a reliable OHKO chance. Additionally, Greninja may opt to use Hidden Power Fire to resist Mega Metagross's Steel attacks, but this leaves it susceptible to being knocked out by Earthquake, thus making the matchup more reliant on predictions if you're unfamiliar with the set or if they're running both Hidden Power Fire and Dark Pulse. Against Altaria, Ice Punch is often the ideal move to start with, as most Altaria will not Mega Evolve immediately out of fear of a Steel-type STAB attack, and try to set up defensively with Cotton Guard; even if they do Mega Evolve, it's still very feasible for Iron Head to pick up the 2HKO on the next turn even if Mega Altaria is at +3 unless they're running a lot of Defense investment. Against Mega Mawile, using either Bullet Punch or Substitute immediately is the best option, as most Mawile wouldn't think to use Play Rough against Mega Metagross, nor can it even reliably break Mega Metagross's Substitute, unless they have near-maximum Attack or Fire Fang. Bullet Punch's utility is to outprioritize Sucker Punch as well as minimize damage taken from a potential Metal Burst. In certain unfavorable situations, like being up against a Zygarde or Air Balloon Heatran, your only option will be to go for the unlikely odds of a 30% Iron Head flinch, which would put Zygarde in range of an Ice Punch 2HKO and pop Heatran's Air Balloon so that you can finish it off with Earthquake. The biggest downside of using Mega Metagross is that it will often find itself needing coverage moves that it doesn't have room for, such as Ice Punch for Dragonite, Zen Headbutt for Kommo-o, and Bullet Punch for Magnezone. Don't be afraid to switch up moves on Mega Metagross to cover whatever weaknesses your team may have at a given point in time.
Team Options
Water-types like Mega Gyarados, Mega Slowbro, Primarina, and Tapu Fini cover Mega Metagross's weaknesses to Fire- and Ground-types like Charizard, Heatran, Donphan, and Garchomp very well, while Mega Metagross in turn aids the Water-types by resisting and tanking hits from the Grass- and Electric-types they're weak to, namely Mega Venusaur, Magnezone, and Zeraora. Stall-oriented Pokemon like Mega Slowbro, Zygarde, Jumpluff, Mega Altaria, and Mega Venusaur work well with Mega Metagross, as it covers many of the most viable Taunt users in the metagame such as Mega Gyarados, Mega Gardevoir, Kommo-o, and Tapu Fini, while most of Mega Metagross's weaknesses are either stall users themselves or bulky offensive Pokemon that have little answer to being stalled down, like Mega Charizard X and Y, Landorus-T, and Donphan. Dragonite is a suitable partner for Mega Metagross, as it can handle the Fire- and Ground-types that threaten Mega Metagross, as well as Z-Conversion Porygon-Z. In return, Mega Metagross aids Dragonite in handling the threatening Fairy-types and Mold Breaker Pokemon that present Dragonite with issues.