A lot of people try Body Press Orthworm which is slow, Skeledirge bulked out which works but has some RNG with the boosted EQ from +2 attack, or Unaware Quagsire with Covert Cloak which is also physical and very slow. Physical ofc slow into Metagross very high defense stat.
You can be immune to everything but Thunder Punch with this build and a special attacker due to Metagross mechanics where it stops using Meteor Mash after it hits +2 attack usually in the first turn, not sure why it works out like that.
Anyway its Stellar Hydreigon. You are immune to EQ and Zen Headbutt leaving weak non stab Thunder Punch which you resist.
Besides Stellar typing its Modest(can go Bold but I like damage into the shield since with Stellar you don't get matching type bonus into the shield cutting damage in half while the shield is up luckly its a small shield), uses the Shell Bell, max IVs besides Atk(and I suppose SpDef doesn't matter if you're building one just for this raid but hey bottlecaps are basically free) and EVs of 252SpAtk/252Def/4HP. Moveset is Reflect, Flamethrower, Nasty Plot, and that's really all that matters.
You start putting Reflect up and take the Meteor Mash shot which hits hard, then you're pretty much set. Nasty Plot a couple times at least and start Flamethrower until Tera and go for the burn to make its damage from Thunder Punch even more meager. Once you crack shield the shield bonus doesn't matter and Metagross is still in green, when he wipes your stats get Reflect up and Nasty Plot x3 then Flamethrower until the end.
Fun build that makes use of Stellar Tera and last time they ran this raid I never lost with it and cleared the raid quickly since the shield is small and its the only time your damage is nerfed significantly but you're still doing enough to get through its smaller shield and heal back with Shell Bell. Once broken Shell Bell will keep you full since Thunder Punch does very little especially with Reflect up. If you get paralyzed just heal cheer and move on. At 10% chance, 4 paralyzes in a match is highly unlikely especially with how fast you clear the health bar post shield. Bookmark or whatever. Metagross event is up tomorrow.
Edit: I suggest Bold nature actually, I think I used it last time but forgot and thought it was Modest. But I ran it twice already with this build this time and while I won both times Bold went much better not needing to burn an extra heal cheer/turn. Just put up three Nasty Plots to break the shield which more than made up for nature switch. I'm gonna try some other builds out including totally base game ones besides Skelly. I'll post any if good. Gonna toy with Mew as well because technically you could get him in the base game if playing then. Got an Earth Power build for him maybe.