It can be pretty hard to keep track of the time of death, since it can be killed by so many people.
Hunting him frustrated me for the longest time, simply because I could not find anyone who could tell me his ToD.
Eventually, I took the advice of other people elsewhere who had successfully gotten the drop for their quest, and I logged out in his room the night before a scheduled server maintenance.
I logged back in as soon as the servers were up the next day (Actually, I arrived an hour late, but I did mean to log in as soon as the server was up), and he popped within 15 minutes.
I got my vine and finished my quest.
We know there are some big updates coming around the end of this month, and I expect there to be a serverdown along with it.
I suggest you bide your time until then, and do what I did.
At the very least, if you fail to claim him when he pops shortly after the server comes up, you'll have been there to note the ToD for the next day.