I am obsessed.
Until my guilds get Critter Kill Squad, I find myself ruthlessly and automatically murdering every rat, cockroach and squirrel I come across. I can't help it! Every tiny little corpse contributes to that achievement. And I don't even want the pet, for crying out loud.
A few days ago I was healing a Heroic as my Priest, and I had my Toxic Wasteling pet out for fun. Now that vanity pets stick around through load screens and logouts, I tend to have one hanging around at all times because, well, why not? I like the Wasteling, since it's fairly uncommon (rare drop from the Love is in the Air bosses), and amusingly, it eats critters and grows bigger with each kill. I used this little slime a lot back in Forge of Souls, because there are a lot of critters in there and I enjoyed seeing my little baby slime nom away and get pretty huge.
My Druid also has the Wasteling! Of course, Rades, my pet collector...does not.
Anyway, I didn't even notice him in this Heroic (I think Shadowfang Keep?) until I glanced over and saw it had grown to half my size. I realized it had been happily devouring critters the whole time, and it occurred to me to check if these counted towards Critter Kill Squad.
Amazingly, they DO! So you could theoretically pop this pet out and just AFK in some critter-laden spot to eventually net the achievement! How awesome is that?
However, this isn't as easy at it seems. While the Wasteling does eat critters all by itself, it's not super fast. I took him to a penguin smorgasbord and watched it work, and it gobbles a critter down every 9-10 seconds. That's 6 every minute, 360 per hour...or 138 hours to get the full achievement. And that's if you're in a place where the Wasteling always has critters within eating range, and you don't have to wait or move for respawns!
That being said, it's still a nice way to just hang out chatting, wait for a DPS queue, or even just go AFK! You'd just be sitting around anyway - may as well whittle away at the achievement at the same time. Personally, I now try to log off each night by AFK-ing out in this manner.
There's actually a perfect place for doing this little trick, though it's significantly less stressful for Horde players. Simply go to Agmar's Hammer in Dragonblight, and head to the southern part of the base. There's a cage with some helpless Scourge in it, some Forsaken chemists, a leper gnome, but more importantly, an unlimited supply of cockroaches. Your Wasteling can just feast away endlessly and the roaches will keep popping up, waiting to be consumed! If you want to actively kill the roaches and speed things up, that works too - they'll instantly respawn, regardless of how fast they're being killed.
Here's a video of the Wasteling in action!
The real advantage of this location is the instant respawns. There's some great critter locations where you can rack up dozens of quick kills, but then have to wait or roam around looking for more. Meanwhile, these cockroaches will keep popping up as fast as you can kill them, providing a never-ending feast for your slimy pal!
According to what I've read, level 85 Alliance players can just fly down next to the cage and they won't be bothered by guards or anything. (EDIT: Aralosseien says that this is FALSE, and that Horde guards will come beat up Alliance players!) Of course, you could also just go to the Deeprun Tram and commit Rat-icide. I've never tested the respawn rate in the Deeprun Tram but judging from my friend's murder spree there a long time ago, they seem endless.
I got disconnected on the Deeprun Tram & had to run back. This is what awaited me.
Finally, I imagine this would work with other vanity pets that kill critters, such as Willy from Orphan's Week, or Lil' K.T. However, I haven't tested with these pets and I don't know how frequently they'll kill a critter. I've only seen Willy do it a handful of times, and I asked on Twitter and it seems Lil' K.T. doesn't do it very often either. So unfortunately, it seems that if you don't have the Toxic Wasteling, you might be out of luck with this little trick.
Oh, and to anyone who has me on Real ID - if you see my Druid (who also has the Wasteling) logged in and in Dragonblight, don't bother messaging me! Chances are I'm not even at the keyboard and won't respond. I'm just working on the achievement. :D