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Aegislash is pretty ridiculous imo. Its King's Shield allows him to tank stuff like EQ's from Choice Scarf Lando, Hydro Pump Specs Keldeo, etc. and then retaliate back with SB dealing a fair amount of 60% or more (and let's not forget about shadow sneak too).
The only counter that I have found...
username: Elainne
Age: 17
Pokemon ShowDown Username: Elainne or l3l
Your timezone and usual hours of availability: (UTC-03:00) Buenos Aires Everyday from 4pm to 9pm
What tier do you want to learn?: OU
Tell us a little about yourself: I started playing Showdown 6 months ago and I would like to...