"Potentially always" sounds weird. Also, it is definitely not a counter to aegislash. It is a check.
Also, it doesn't kill togekiss, one of the best fairies. Togekiss outspeeds and OHKOs with aura sphere. It can't be sure to beat azumarill either.
LO 252 Attack Adamant Bisharp can OHKO Shield Forme Aegislash with Night Slash if it comes in on Sticky Web, which could very well be present if the opponent's using stuff that isn't Aegislash. That is taken from earlier in the thread. I don't remember exactly how much Sticky Web lowers Speed, but I can figure out how many Speed EVs an Adamant Bisharp would have to run to outspeed a standard Brave 0/0 Aegislash for certain values. Aegislash's Speed is 112, so we want to solve for 113 in our stat formula.
Sticky Web is -1 Speed (2/3):
113 = (((31 + 140 + x/4) * 100)/ 100 + 5) * 1 * 2/3
113 = ((171 + x/4) + 5) * 2/3
169.5 = (171 + x/4) + 5
164.5 = 171 + x/4
-6.5 = x/4
x = -26 EVs
Always.
Sticky Web is -2 Speed (1/2):
113 = ((171 + x/4) + 5) * 1/2
226 = (171 + x/4) + 5
221 = 171 + x/4
50 = x/4
x = 200
200 Speed EVs.
It's not more than 2 stages, I know.
So Bisharp, with 452 out of its 508 usable EVs, can OHKO Aegislash given Sticky Web on its side of the field.
Unfortunately, it can't survive a Sacred Sword and, therefore, without Sticky Web, OHKOs with a crit Night Slash (which happens at least 1/8 of the time). So, if you can Baton Pass a Swords Dance to Bisharp OR switch it in on an Aegislash with Sticky Web, it kills Aegislash. That makes it among the best counters that guy's got.