I have mixed feelings about this. On one hand, I noticed that if someone helps you kill a monster, both of you get credited for the kill. I know this because it's happened both ways for me:
- I joined in on a monster someone else was attacking, and was credited with the kill in my hunting log/quest requirements, despite me not striking the last blow.
- Someone else joined in on a monster I was halfway killing, and I was credited with the kill as well.

I believe you have to do enough damage to it, to be credited with the kill.

On the other hand, I am just simply insecure about not getting my credited kill, because someone else joined in on my monster. Just disproved insecurity, and nothing more however.

I've not seen anyone called out on this however, so I assume it's something perfectly acceptable...