Not entirely correct, there’s also an achieve for killing it at all , which is doable in a group.
But you only get the golden Skin for teh mount and the solo achieve if you, of course, solo it. And that´s why most players are taking it on solo or farming gear until they believe they´re ready (which actually won´t help all that much much because it´s a pure mechanics fight… play the mechanics clean and it eventually dies, ignore /fail them too often, and it won´t.)
That is what´s generally referred to as “normal” mode, because it´s the easiest difficulty of the fight.
But, again, it`s primarily a mechanics fight, not a DpS race. And ignoring that and trying to go full supersaiyan instead is generally what causes most people to fail IME, just as it was with Zek’Vir.
FWIW, here’s a guide for the ?? version, but everthing relevant incl the basic tactics is exactly the same as ?. Maybe there’s just something you’re missing /overlooking. Good luck on your next attempts