2mil Normal
20mil Nightmare

at level 98..

I always thought saving the Hell Ancients was difficult due to Baaling and what not; but in this case if you're rushed you can head back and snag these quests for some hella awesome XP.

Not sure if it translates to D2:R or not, or why it took me over a decade to realize this.

some people actually did this in D2L to optimize their experience gain, but in D2R this method seems to no longer be useful due to terror zones providing devent exp also at the top levels