From wowhead:
"Just a tip to add on to the "killing blow" requirement; just like healers will want to try and solo for this to ensure they get the last hit, hunters and warlocks (and other pet-owners) will also want to solo without their pets. I didn't get any charms until I dismissed mine."
"Yeah, I've found that they don't drop if my pet gets the killing blow. The way I'm working around it is to get the pet doing most of the work until the mob is low(ish) on health, and then quickly setting the pet to passive whilst I finish the mob off to make sure I get the killing blow..... it's less hit and miss than timing the 'kill shot' - although I do generally use the signal for 'kill shot' available (mob is 20% of it's starting health) as the timer for setting the pet to passive - that way if the pet has a bleed effect active, it generally runs out before i deliver the killing blow. T"
Can anyone confirm if we need the kb to get one?