Isle of Thunder

Unlike the armor appearances, you only need to do one of the achievements here to unlock the weapon appearances. Your mileage may vary depending on zone congestion, but I was able to get this done pretty quickly just by pulling up a map of the rares in the zone (note, you don't want the ones you spawn with Shan'ze Ritual Stones, but rather the ones that spawn naturally) and running around until I got three of them. Try to call out spawns in map chat because there are no doubt going to be a few other people in the zone looking to do the same thing and we all just want to get the achievement done.

Timerunners access to Isle of Thunder starts at level 50, from a faction quest near Shadow-Pan Garrison in Townlong Steppes.

The probably fastest way to get this is just by doing the campaign solo instances.