This guild achievement is achieved when the guild kills a total of 50,000 critters.
A few tactics of how this can be achieved fast.
- There is a tunnel in north-western Eastern Plaguelands that is full of critters, the respawn time isn't very good though.
- The Tundra penguins in Northrend are a good farm place and respawn quick.
- In the Worgen starting zone there is a tunnel where critters keep respawning, you can easily get thousands of critters an hour, some say 10k/hour.