-Mandibuzz
-Chesnaught
-specially defensive Hippowdon
-Various stuff with STAB Earthquake and Assault Vest, like Lando-T
These are generally the best counters, but Aegislash has a set that beats all of them. Max speed Aegislash with LO can 2HKO Mandibuzz after stealth rock, and Chesnaught without any prior damage. Hippowdon can be toxic stalled. Ballon Aegislash is also common, and it can run HP Ice to OHKO Lando-T and various other ground types (only with Life Orb though, so it's exclusive to balloon Aegislash.) It's about as close to no counters are you're going to get.
It's fun when pic related happens, though.
>>18564424But you do start running into Toxicstall, Flash Cannon and HP Ice swords.
