The challenge seems to require that there is only ever one player inside the instance at any time. However, one player can clear all the trash and Domina Venomblade, zone out, and then another player can zone in to defeat Margrave Stradama.
Continue reading ยป
After tweaking with the challenge for a day, players from the Secret Finding Discord community have discovered a method of helping others complete the challenge to obtain the Slime Serpent mount.