What they should do for this quest is up the chances of it spawning each round it doesn't show up. And when it does but you don't get the kill, the game'll keep track of how many times you go at it. If you don't kill it like the third or so time around, the Drifter will call you over and be like... "Hey there, Guardian. Saw ya throwin' down with that big ball of nasty in the arena a few times now. Never seen anything like it before, but I've got a bead on what calls it out. If ya want, I can draw it out just for you and your crew, cuz let's be honest here...we both want to see what that thing has in it. I'm bettin' it'd be worth the effort for both of us!"