I had one of these drop after killing less than 10 Broken Skull Guardians. This was an easy solo hunt as a 57 ranger, with the guardians being dark blue to light blue. Invis in from the Gunthak zoneline, and take the street to your right. The street ends in a railing overlooking the ocean -- from there, make a right turn and camp where the railing and the rock wall meet, and you'll get no adds. The guardians are the only wandering mobs that I've noticed in the zone, and are all trolls. Their path takes them up and down the middle of the streets. You can just settle in, wait for one to pass by, drag him back into your cul-de-sac, and kill him in safety.