You don't need to use garbage sets or garbage pokemon to handle Metagross.
that doesn't say much. mega salamance is beyond megagross just as salamance is beyond greninja. both of the lesser threats are still too much for OU.
It only has two STABs???
not sure what you're trying to say here. yes? i was saying that most charizard sets that ran EQ were forgoing one of its STABs (generally fire), therefore if you include its ground coverage in your list of mons it 2hko's, you have to remember that some of the mons it would previously 2hko with, say fire punch/flare blits, are no longer 2hko'd by it.
Slowbro counters Metagross so long as it has some SDef investment (156+ for 0% chance of 3HKO after SR.) You literally CAN put Slowbro on your team and "bam." Or Slowking. Or you can run fully defensive Slowbro and run something that can check sets that don't carry Ice Punch. When I say "stack checks to it" I do not literally mean dedicate half your team to checking it, I mean that if you find that your team is weak to Metagross then you make adjustments, like you would other top threats.
0 SpA Tough Claws Mega Metagross Grass Knot (80 BP) vs. 252 HP / 252+ SpD Slowbro: 134-158 (34 - 40.1%) -- guaranteed 3HKO
better hope you get that burn on your one scald. also i'm failing to see slowbro avoid the 3hko? unless of course you suggest that i must run mega slowbro to avoid the 3hko
0 SpA Tough Claws Mega Metagross Grass Knot (100 BP) vs. 252 HP / 252+ SpD Mega Slowbro: 166-196 (42.1 - 49.7%) -- guaranteed 3HKO
so now with rocks up you actually do get the 2hko on mega assuming you get more than 2 min rolls in a row. and if it's regular max spdef max hp (garbage) you just barely miss the 2hko with rocks up but that's fine because slack off right? sure mega slowbro could take it with spdef boosts but that's about it. it's not a switch in to Grass Knot sets. nice counter (which is way worse than its physically defensive counterpart. if that's not using suboptimal sets to beat it, i don't know what is).
Things like Rotom-W and Suicune can "switch in" (Rotom is 2HKOed tho) but then what? What do they do back? X-zard can heal damage. Metagross can't. And lol at "if it's not running Fire STAB." I mean if you're listing Ferrothorn as anything that resembles "does not get BTFO by majority of X-zard's", then I think I'm fine listing Jirachi, Victini, Slowking/SDef Slowbro, Hippowdon, Skarmory, Ferrothorn, bulky Garchomp, Scizor, Starmie, Gliscor, etc. etc. as checks to Mega Metagross. Because they are.
you're right, the notice i made about it the calculator failing to account for things that help my argument, it also fails to consider whether or not these switch-ins can do anything in return. but couldn't rotom run t-wave to cripple it? or suicune toxic to put it on a timer? less than optimal but sure as hell a lot more viable than spdef slowbro imo. also the argument was about zard-x in XY. at the end of the day megagross can bust up your answers depending on what it puts in the last 2 slots. the only truly necessary moves are MM and ZH. HA is arguable for steels but you could run EQ and a mag to trap steels so i don't think it's truly necessary like dual STAB. zard did not have this luxury. don't get me wrong it had various sets like any other top tier threat. but bulky will-o just isn't as effective as DD, so there goes a slot on DD. you could forego roost, but i wouldn't recommend it. give a mon that powerful with its ability to clean up / sweep reliable recovery and you take it. 2 slots left. depending what you pick you're hardwalled by one of the 2 most popular rock-setters in the game, each of which can fuck your day up if you don't pack the coverage type for it and can set up rocks on you preventing you from switching in nicely later. i suppose you could forego dragon coverage but then imagine how many other things wall it. you could go 3 attacks but that inherently implies that zard needs much more team coverage in order to perform so that it remains healthy throughout the match and can come in without rocks up. or suppose you get rid of DD, then you're not hitting the speed tier necessary to be as threatening as zard was in the first place. this inherently makes zard less effective than megagross because this does not apply to him. he picks his stabs, picks his coverage, comes in and dents. it's more similar to greninja in that it can pick its checks to stop and then the team can patch up the other (obvious switch-ins) that megagross has.
Did you think that X-zard was as busted around a year ago, in the early XY meta?
not really, no. slowbro, talonflame, lando, and azu are all random ou mons i could pick off the top of my head and patch up my weakness to zard. with megagross i have to use bulky chomp or slowking apparently. and a lot of the ones that would come into my head that are actually good like jirachi just get bopped by the right coverage move.