Mandibuzz is the only true counter for aegislash but who in their right minds would want to bring a mandibuzz into an OU match? Same with Cofagrigus and Dusclops. The latter two are admittedly better pokemon than mandibuzz, but you are gimping your team overall by bringing one in a match. Unless every freaking team has a kangaskhan, cofagrigus and dusclops are liabilities. You're pretty much making it a 5.5 vs 6 match if they lack a kangaskhan. You're much better off using mega lucario, skarmory, or the grass/ghosts to OHKO or phase him out.