The only times my Terrible Turnip
's weakening blow has ever killed a pet was when one of the following was also true:
1) I had a DOT on the pet
2) There was some weather condition active (such as Scorched Earth or Lightning) that caused additional damage
3) The pet had some ability (such as Stone Rush) that caused it to take damage when it used it
While weakening blow cannot in and of itself kill a pet, if it is combined with some other damage source, the pet will die.
Edit: Ok, after reading your second post, I am going to have to do some experimenting to see if I can duplicate your experience.
Edit #2: Ok, I took a team to the Dalaran Crater. I found 2 Rat
s and 1 Lofty Libram
. I engaged each with my team. The first rat went from full health to one hit point in one blow (I skipped using my lowbie pet against that one). I was unable to do any more damage with that ability. After trying a few rounds, I hit it with leach seed and killed it. The Lofty Libram, I also started out not using my lowbie pet. Since they are magic, it took 3 hits to bring it to 1 health. After a few rounds, hitting it with weakening blow, I forfeited the match and re-engaged it, this time I had my lowbie hit it first. I had the same result. Since it was a common and I already have a rare, I finally killed it with leach seed. The battle with the second rat went much the same way as the first had.
Question: Exactly what pet were you leading with and what specific ability did it use?