Hey, I have just done this with my guild, after one of the officers thought it would work. Please read on.
Warning: This will require a group from your guild. The more, the faster.
Theory: If you kill one critter, with 10 people in a raid group from your guild. It will count as 10. As everyone gets the credit. So we did some maths. One AoE Spell caster could kill around 2,000 critters in an hour. So if you have 10 people in your group. You get credited for 20,000 critters killed. And the same goes if you have 20 people and so on.
I recorded the whole thing with my guild. And watch how fast we got 12K. Well under and hour if you include travel time. We first tried out the cave in the Plaguelands. And we had an issue with spawns. However, when me and one of my officers tried the penguins, we found. If you killed one, another one will spawn on a different island. So. If you put a group on all 3 of those islands. You get it done 3x the speed. And instant respawns. This is how it was done.
If you are Alliance, you can do this quite easily on the IF end of the tram. The rat respawn rate is incredibly high. Get a group together (even guild members who may be going afk, (put them in a raid group in the vicinity) and kill. You can get to 50k very quickly with a couple of AoE classes.