I've been in situations like that many times, and I usually just find the biggest group of mobs clustered together I can find to feign like I am about to pull them and interrupt myself at the end... if he tries to "steal" this kill he'll be surprised to find he got all the aggro to himself... While he's busy with that, I finish myquest up and leave.

On the other hand, I frequently help people that are doing the same kill quests I am, I just tell them to mark what they need and I burn them down for a few kills, or I'll just shock their mob as they are fighting them as I run by... No reason to drop one of your 5...