Critter Gitter

I highly recommend the penguins off the coast of either Howling Fjord or Borean Tundra. You won't need a flying mount if you've got Elixir of Water Walking or a class ability to help you swim... or a lot of patience to swim out at normal speed.

Not only are there 15+ penguins in a group, but they stand still so they're very easy to target. I completed the achievement in about as long as it took to do 10 global cooldowns.

While exploring Icecrown today, I found an excellent spot to do this achievement. In the very northeast area of the zone, there are large flocks of penguins. It was about as easy as it gets. Screenshot is posted.

Here is a link to the penguins:
http://www.wowhead.com/?npc=32498

For Alliance I would suggest going to the cat lady's house in Goldshire. Plenty of cats run around inside and outside her house, along with some cows. Was very easy for me to get the achievement this way.