Act 3 Quest 6: The Guardian - Diablo 2
Quest Location: Durance of Hate
Quest NPC: Ormus
Quest Goal: Defeat Mephisto
Reward: Mephisto's Soulstone & Act 4 Access
Flag: Access to Act IV
You receive this quest automatically the first time you step foot into the Durance of Hate. Basically all you have to do for this quest is make it down to the Durance of Hate level 3 and take out Mephisto, the end boss of Act 3. In order to gain access to the Durance of Hate you'll first need to complete Act 3 Quest 3: Khalim's Will as well as Act 3 Quest 5: The Blackened Temple. Both of these quests complete the moment you destroy the Compelling Orb next to the Durance of Hate's entrance.
The Durance of Hate makes a very good farming location since the entire dungeon is level 83 in Hell which is one of the highest area levels in the game (meaning it can drop almost every item in the game). In addition to this, the Durance of Hate Level 2 is one of the largest zones in the entire game which is great for farming but terrible if all you want to do is get to Mephisto and take him out.
If you'd like to learn more about farming the Durance of Hate I recommend heading over to my Durance of Hate Farming Guide which will tell you all you need to know about this dungeon. Additionally, if you'd like to learn more about farming Mephisto check out my Mephisto Farming Guide. Mephisto is one of the most commonly farmed bosses on Hell due to how easy he is and his drop table.
Classes or builds that rely on Melee to deal their damage will have the most difficult time with Mephisto since they'll be unable to exploit the moat trick or any other ranged/kiting method to beat him. Mephisto is a caster and has some of the worst pathing imaginable due to the layout of his room which makes it very easy to get him stuck just about anywhere.
This concept has actually been perfected with the 'moat trick' (pictured below). If you're someone with the ability to Teleport you can basically run up, agro Mephisto, kite him down his stairs a little bit and then Teleport across the moat to the other side. You can then effectively stand in this location and Mephisto can't reach/is too far to use most of his abilities on you. So he'll just run into the stairs (as he is doing in the screenshot below at the top left of the picture).
Once you have Mephisto stuck in this spot you can AoE the edge of the stairs in front of him which will damage him. Using this trick you'll be able to defeat Mephisto and barely take any damage; which is exactly why this is the #1 method for getting gear at the start of each new Ladder. Since it's so safe you don't need good gear to do it & you can effectively load up on the best Magic Find gear to farm here which will get you better loot.