The most efficient way to level a fresh character to level 15 is Tristam runs.
What is a Tristam Run?
Tristam runs consists of going to the town of Tristam (throw back to Diablo) and clearing every enemy in the town. In doing so, you will clear several rare and a unique boss which will give you a substantial amount of experience.
In terms of best bang for your buck, from a time to get to and kill enemies perspective, Tristam runs are it until level 15. You are guaranteed a certain amount of rare enemies and champions every run.
On top of a small map and generally easy to discover entrance, it’s a perfect scenario for farming.
What is the Most Efficient Way to do Tristam Runs?
Tristam runs are most efficient with eight people. This will capitalize on your experience gained and is easy to accomplish.
If you go into the lobby and look for anything with the phrase “trist” in it, you are bound to find a lobby that will keep farming.
Most lobbies will keep the same base name. For example “TristWalk03” will become “TristWalk04” once the map is cleared. The name of the game here is to clear the map quick and move onto the next game.
This obviously will not be all that much fun, and you will not be focusing on loot here, but if your goal is to level as quickly as possible, this is it.

Wrapping Up
I recently rolled my second character in Diablo II Resurrected after putting far too many hours into a necromancer. I was able to hit level 15 in less than an hour using Tristam runs.
So I can definitely stand by my claims here.
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