Your right about aegislash trolling you with king shield but even if you attack with icicle spear you're still beaten.Aegislash is just going to troll Cloyster with King's Shield if you attempt to use Razor Shell on it.
It's best to go for the 2HKO with Icicle Spear and leave that moveslot for Ice Shard.
+2 252+ Atk Life Orb Cloyster Icicle Spear (5 hits) vs. 252 HP / 0 Def Aegislash-Shield: 160-195 (49.3 - 60.1%) -- approx. 89.5% chance to 2HKO after Leftovers recovery
While aegislash will just sacred sword then shadow sneak leaving cloyster ko.