As you finished Quest 5, the door beside the Durance of Hate will open. Find the final Boss Mephisto at the third floor of Act 3. Before reaching the third floor, here I recommend walking counterclockwise along the edge of the map, and finding a waypoint when you are in the second floor.
The damage for Mephisto are extremely high that will cause death of your character, so waypoint can help shorten distance and time. Monsters in the former two floors are already strengthened. You'd better be carefully so that not be surrounded by them. Handle the three mini-boss Council Members before attacking Dragonhand on the third floor. Your mercenaries can be used to ward off most damage for you during the Fight with Mephisto and Maffer Dragonhand. Replenish your status by making regularly use of teleport. After Mephisto is dealt with, pick up your loot and his Soulstone and then head through the red portal that opens up to move to Act IV.
How to complete Act 4 Quest 1: The Fallen Angel?
The towns in Act 4 are smaller than previous towns where you can find the four NPCs easier. Talk with them to get new tasks. In fact, Act 4 is relatively compact in general. You only complete three tasks. While you are looking for core gears, D2R items that can increase resist are also included. Monsters in Act 4 will cause a lot of lightning and fire damage. The desert mercenary is suggested to use due to the ability to survive in some of Diablo's attacks. After preparation, talk to Tyrael to take up the first task.
The first task is relatively easy, that is to kill the Fallen Angel Izual. You need reach the Plains of Despair from the Outer Steppes and find it first. There appears plenty of new monsters on the way. Among them, the Burning Soul need to be attended. Then find Izual at the edge of the Plains of Despair. If the character is Sorceress, some mana potion are required. Although the damage is not high, it takes a long time to kill him. Back to the town and conversation with Tyrael, you'll get the rewards: +2 skill points.
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Article from: How to complete Diablo 2 Resurrected Act 3 Quest 6: The Guardian and leave for Act 4?