Use Weighted Jack-o'-Lanterns to put pumpkin heads on each of the races listed below.
Weighted Jack-o'-Lantern can also be purchased from the Auction House because the item does not require Hallow's End. This means that the achievement itself can be earned when the world event is not active.
Note: I play on PvE realms. PvP-realm characters may need to adjust accordingly.For low-level characters: Your best bet for getting the Horde races is outside of Orgrimmar. There are plenty of players dueling outside of the Gates. The same is true of Alliance characters outside of Stormwind; there are also plenty of players in and around Goldshire, but you will need to take added care with the guards.
For mid-level characters: Hit up Shattrath and Dalaran. These Sanctuary cities still have life in them. You may still need to visit near Orgrimmar or Stormwind if you just can't catch a particular race.
For high to max level characters: Timeless Isle because it is current content. Hitting up your faction's shrine is not as useful because you won't encounter (well, unless the person is exploiting a glitch) races of the opposing faction. If you are still missing a race or two, then try the tips for low- and mid-level characters.
Use Weighted Jack-o'-Lanterns on one of each of the races lists in the achievement.
Weighted Jack-o'-Lanterns can be obtained in the following ways: