I changed tactics multiple times, but here's what worked and some of my mistakes I think. Maybe I just finally got lucky:
Kill the elephants also. I'm not sure they are PHs, but if they are, you'll never get him.
Stay in the area where Mroon spawns and slaughter it all. It may take an hour for the PH to path back there, but it eventually will. Previously I was roaming back and forth from the aviaks to the eastern zone line, and I think missing a lot of PHs, who then roamed off for another hour. Roam 100 yards or less. Don't let a single mob escape that area alive.