This specific challenge is listed as follows: Reptile Displacement: Kill the Ancient Croc. There are no qualifiers about when you have to do it or how you have to do it; just that you have to do it. I did all the steps in the mission except opening the chest. Then I went found the ancient croc and killed it. But the challenge did not get marked as completed. If it is intended that you have to kill the ancient croc before the final boss, then you need to put that in the achievement description, otherwise just killing it at any point while in the zone should tick the box.
Def needs to be done before the chest pops. I ran into the same issue first attempt and didnt get credit for croc. Second time through beelined NE to the croc, killed that and did the rest of the mission. Credit was given for kill that time.
Confirming this. In my case, I killed the ancient croc as soon as I zoned in. But the achievement did not register until after I killed the final named. A possible related bug is that the quest journal stopped updating after I got the third island update. Despite having killed the mobs on the third island, the quest journal got stuck at 0/2 mobs. Nevertheless, the final named still popped after the third island was cleared and a chest appeared once the named was dead.
My point is that the Achievement should either allow you to kill the ancient croc at any point as long as you are in the mission or be changed to specify that you need to kill it before you kill the final boss of the mission.
