Metagross, Snowballing Sweeper of Classics.
Overview
With full unlocks, Beldum is an excellent starter pick for Classic, from Wave 1 to 200. It operates as an immediate physical sweeper at all points in the game.
Strengths
The Beldum line has an excellent defensive typing, solid bulk, and several moves that boost its physical stats. This allows it time to set up with Shift Gear and destroy almost any opponent that could stand in its way. Its egg moves provide strong coverage for early levels, and its stat boosts make it almost impossible to remove on the physical side and most non-bosses unable to survive more than a hit against it. Unlike most physical attackers, Clear Body also renders Metagross immune to stat drops from Intimidate, letting it maintain its snowball more easily. It's strong against most Mega Rayquazas in particular, being more than tanky enough to survive its attacks and out-damage Ray unless it rolls Fire coverage.
Weaknesses
Metagross' best moves have 90 accuracy, making its offense a bit unreliable, and able to miss at key moments. It also lacks a way to boost its more modest SpD on its own, leaving Metagross vulnerable to getting sniped by super effective SpA moves. Metagross generally wants some support when fighting Eternatus. Metagross is rarely tanky enough to survive more than one or two of its Flamethrowers, preventing it from having the time to effectively set up. As a physical sweeper without Guts, Metagross also hates running into burn.
Recommended build
Metagross can run either Adamant or Jolly just fine, though benefits more from the latter to outspeed more key threats. Beldum likes to run all 4 egg moves over its lackluster move pool. Metagross has a few options, but generally runs best with Psyshield Bash, Meteor Mash, Drain Punch, and Shift Gear. This gives a setup move, 2 physical STABs that can boost its physical stats, and coverage that doubles as a heal. Drain Punch is generally better to keep over Ice Hammer due to the greater consistency, healing value, and providing crucial coverage against the Dark types that like to switch into Big M. Wide Lenses, Leftovers, and a stock of Lum Berries also help add consistency to its sweeping. Mega is also a straight upgrade, replacing Iron Fist with the superior Tough Claws, meaning Metagrossite should always be prioritized if found.
Moves & abilities
Shift Gear: Essential setup move. Boosting Atk and Spe makes Metagross go from strong to a snowballing menace. Psyshield Bash: 90 power Psychic move that always boosts Def. Easily spammable to make Metagross increasingly difficult to remove from the field. Meteor Mash: Metagross' quasi-signature move, a 90 power Steel move with 20% chance to boost Atk. Slightly stronger than Psyshield with Iron Fist, and the occasional boost is appreciated. For TMs, Iron Head can be chosen to swap some damage for consistency/flinch chance, and Heavy Slam usually hits harder due to Metagross' massive weight. Drain Punch: 75 power Fighting move that heals, boosted by Iron Fist. Provides key coverage against the Dark types immune to Psyshield and recovery to a mon otherwise lacking it. Ice Hammer: 100 power Ice move that lowers Metagross' Spe, boosted by Iron Fist. Only used early in the run to provide supereffective coverage against Flying and Grass types in the early routes. Can be replaced with Metal Claw, Iron Head, or Meteor Mash depending on coverage on other teammates. If some egg moves are missing, Metagross still thrives just fine while using Zen Headbutt, Psychic Fangs, and/or Hammer Arm. If Shift Gear is missing, running a 4th coverage move is fine, although it severely hurts Metagross' ability to snowball. Metagross always wants to run Clear Body over Light Metal. Immunity to stat drops is essential for a snowballing physical attacker fighting against Intimidate.
Strategy
The strategy is simple. Start with Beldum's four egg moves, Clear Body, and Even early on, Beldum usually survives long enough to use Shift Gear once or twice, and prefers to use Psyshield Bash whenever possible to bolster Def. After the stats have increased a few times, Psyshield, Mash/Hammer, or Drain can be alternated depending on what faints the enemy fastest. If all damage types are neutral, Mash will hit hardest due to its STAB and Iron Fist passive. The AI often favors switching Dark types into Metagross due to its immunity to Psychic, so many trainers can be baited into switching in on Drain Punch to either instantly knock out mons that otherwise threaten Big M or deal huge damage to them with a free heal. As a snowballer, Metagross prefers to be on the field as much as possible and switching out as little as possible. If the opponent opens with a strong special attacker that hits its weaknesses, it's usually worth opening with another mon to deal with the threat and switch Metagross in afterward, though Metagross can often deal with these mons after a bit of setup. In double battles, Metagross can be paired with mons that redirect attention or draw in attacks to give it time to setup. If a Map is found, it's best to avoid areas where Fire or Ghost types are common, since they hit Metagross super effectively and are not especially weak to the main moves on the set. Dark and Ground-focused areas can be troublesome, but Drain Punch will usually be enough to deal with Dark types and most threatening Ground types are physical attackers, giving Metagross more time to set up and sweep.
Synergies
Metagross benefits from a bit of support against its weaknesses, mostly Fire and Ghost types. As such, any capable check to them is appreciated. Water and Dark types especially help, due to Water hard checking Fire/hitting Ground types hard and Dark hard checking Ghost while resisting itself. Mons that specialize in redirection or support through bolstering defenses can also be of huge help, giving Metagross extra time to set up. Mons with access to Drizzle can especially help with covering those weaknesses. Metagross can defeat Eternatus with some help or enough vitamins, but it appreciates a reliable DoT setter to help trivialize the fight.
Tips for beginners
When running sweepers, invest most available resources and battle time into them. Using Metagross often in fights helps grind candies on it if any unlocks are missing, and investing as many relevant vitamins and berries into it as possible just make it better and more consistent. This is especially true for Lum berries, which help prevent Metagross from suffering burns that half its damage output.
Tips for advanced players
If you know the current trainer has a strong Dark type on their team and Metagross is out, use Drain Punch. The trainer heavily favors switching into Dark types against Metagross, and it provides a free opportunity to instantly get rid of a mon which could otherwise be a huge bother.