I just recently managed to confirm that this works, when I switched guilds to one that still had this achievement open. Anyway, its not particular noteworthy, but some people might find it useful to know, so I might as well share it. Did a forum search as well, and couldn't find anything about this, but might have missed something. If so, my apologies.
Anyway, here we go:
When trying to complete the achievement Critter Kill Squad I found out that your Creepy Crate can help you with this achievement. While most of you probably already knew this, it also helps to know that it kills friendly critters as well. An example I found was Zena (Wodin's Warrior Kitten) at the Crucible of Carnage. Creepy Crate will just gobble her up, adding another critter killed to your count. Now what's even more interesting is that Zena has a respawn timer of about 5 seconds. And every time she re spawns, Creepy Crate will just gobble her up again.
So basically, you can just park your character here, go AFK, and slowly but certainly work towards the Critter Kill Squad achievement. Sure, its not going to go at breakneck speed, and there are other ways that are faster, but this is one of the few ways that'll let you work on it while AFK.
While not confirmed, I do not doubt that this'll work for other pets that kill critters as well. I only have the Creepy Crate, but I'm sure Lil' K.T. and others like him will also do the trick. Also, other spots with fast re spawning critters will obviously also work.
Extra info:
Crucible of Carnage location: Twilight Highlands 50-58.
Anyway, just a lil' titbit of information that some people might find useful, nothing big, but I hope it helps some people.