Karxrida landorus-t is mentioned twice in checks and counters, victini is a meh answer because eq is still common on mega metagross. if you're going to comment that eq is not a good option, it's certainly not the best option available (hammer arm is), but an ohko on heatran, doing ~60% to other metagross, a chance to ohko zard x after rocks, a clean 2hko on jirachi, and no speed drops certainly don't make it a bad option and as i said it's very common. victini can check hammer arm versions though.
you do need to add physically defensive gliscor (not the xy fast versions, but actually defensive versions) as a decent answer to non-ice punch versions bar meteor mash attack boosts. also, physically defensive mew is a counter, mega manectric is an ok check, resttalk gyarados with enough evs to avoid the 2hko from zen headbutt after rocks (248/104) is a solid check and very solid counter if it runs an even more physically defensive spread, suicune does ok against non-grass knot variants (with no rocks on the field, zen headbutt is a 4hko), and gastrodon beats non-grass knot variants. i suppose dugtrio is also worth mentioning because it can trap and 2hko with eq, though it completely loses to bullet punch variants but those aren't /that/ common.
mega sableye is not really a counter, it has a chance to be 2hkoed (albeit very small at 4.7%) after stealth rock damage and this means it loses with basically any amount of prior damage. mandibuzz has to run 252 defense to beat metagross with rocks on the field, which is typically not what it runs (it likes to run some sdef to handle landorus-i) so make sure you specify that. also mention that skarmory should run a 29 spe iv to underspeed mega metagross after 2 hammer arms, this lets it beat it _much_ more effectively.