They did not even give a realm first achievement for Lei Shen, only Ra-Den, so Blizzard must be just a little bit confused or inconsistent on what they think is the ultimate kill for the teir.
All ranking sites are 13/13.
A limited attempt boss gets more respect from me, tbh. Killing it in the least attempts is more impressive than drinking caffeine and grinding out 400 attempts over a week.
I have no doubt whoever gets 13/13 first takes the teir. Plus, I want to see if he drops a mount!