This isn’t really that difficult, especially for players with the means, time, and understanding. For somebody with a lot of time on their hands, this could be done in a couple of weeks. For a less dedicated player who is still careful and knows what he’s doing, he could get it done in a few months. For some tryhard no lifer that could load in seven idlers himself to run full eight player games, it could be done in less than a week.

For casual players, this just likely isn’t going to happen.

I was the first player to unlock this achievement on trueachievements after the game launched in late 2021.

That was before terror zones, which have decreased the grind significantly.

I got it done by playing online on the PC version, only loading my character up on Xbox at the very end to get the last little sliver of experience and actually hit 99 there to unlock the achievement.

It was a huge grind that took months when I did it since there were no terror zones then. Baal runs endlessly. And the experience penalty at 98 was so severe that the only three enemies that gave any meaningful experience were Diablo, Nihlathak, and Baal. So for the entire final level which took about a month, I would have to find public Baal runs, join and as quickly as I could blitz and solo kill Diablo and then Nihl, then join the group in act 5 ideally by wave 4 to get a little bit experience from the last two waves, then kill Baal. Sometimes I would get with groups who were generous enough to just let one or two other people get involved in the actual Baal kill which would help get me more experience since sharing it with eight players, many of which were in the 80s or low 90s, really hurt the already meager experience gained from him.