I'm hesitant about calling Skarmory a counter. I mean, it can phase it out, but it doesn't really pose an immediate threat. Brave Bird takes like 1/4th of Arceus's HP, while +2 ExtremeSpeed does 35.92 - 41.91% to SpD Skarmory. +6 ExtremeSpeed does 71.25 - 84.13%. I mean its a very solid check, but if Extreme Killer is the last mon you can't really count on Skarmory unless you are running something weird like Toxic Skarmory or have Toxic Spikes support and you can phase it out after/if it used a Lum Berry.
I DONT DESERVE YOU