When I am completing a Hunting Task or Killing in the name of quest especially at higher levels I am always with the my tank. The magic number to that question is 51%. So if you are hunting Hydras with a partner or solo, in order for it to count in your tasks is you have to personally over 50% of 2350 Health. So who ever does 1176 damage. If there are three or more players, the person with the majority damage gets the kill.
Only thing I can not answer off the top of my head is if someone had previously damaged the creature and logged off. I believe the rule of thumb is if you would have gotten the loot being solo, you would get the kill count.