Aegislash seems very threatening to the two Pokemon we have so far. Gardevoir is weak to Ghost and steel, and can't really dent 'Slash with the current moveset. Bisharp fares better, getting off a super-effective dark type move, but it is easily OHKO'd by Sacred Sword. Given how strong 'Slash is in the current metagame, a counter is needed.
The solution?
;479-w;
Rotom Wash is, in my experience, one of the best counters to Aegislash in the game. Will-O-Wisp cripples it completely, and then 'Slash goes down after a couple of Hydro Pumps or T-Bolts. Rotom-W also boasts the ability to take down most physical attackers with its good typing and Will-O-Wisp. The bulk it brings is a nice balance to the power and relative frailty of Gardevoir and Bisharp, stopping sweepers before they can kill our biggest threats. The specific set is this:
Physical tank Rotom
Item: Lefties
Nature: Bold (+Def, -Atk)
EVs: 252 HP/168 Def/88 Spd
Hydro Pump
Thunderbolt
Will-O-Wisp
Substitute
This set is designed to tank physical hits like a champion. Hydro Pump and Thunderbolt give Rotom good STAB coverage. Will-O-Wisp cripples physical attackers and slowly kills them. Substitute can be set up before knocking out one pokemon so that you get a free hit on the next. This is particularly useful if you use the free hit to land a Will-O-Wisp, enabling a safer switch-in for your next pokemon. It also allows Rotom to act as a pseudo-wallbreaker: it stops non-attacking moves from hitting him, and he's bulky enough that the sub will probably last a couple of turns, allowing you to wait for them to die of burns if they don't switch out beforehand.
The EVs in speed allow Rotom to outrun threats such as Scizor, meaning that he can get in a Will-O-Wisp or Substitute before they can do anything about it. The rest of the EVs are to make him as bulky as possible.