That also requires RNG to be on your side because those panthers hurt quite a lot if you eat the damage even if you face them one by one. Unless your defense somehow exceeds their accuracy, I guess. I'm assuming people going for triple crown solo just skip that fight.
They are actually skippable - entering main dungeon makes them disappear and they are grouped far enough from the door that it could be done with almost 0 Stealth skill.
Same with Gorrecci Street, quest could be done without any fighting.
It is kinda most ridiculous thing about Ultimate - easiest way to do it is ultra-pacifist run until you lvl 17-18 and only then go through the list with kills needed for completion.
I still has a suspicion that guy who brags about killing them solo did it with full set of Berath Blessings that allows lvl 4 MC at this point.