In order to complete the Mandalorian Beskar quest in Fortnite Chapter 2 Season 5, you must defeat the Mythic Boss, Ruckus. Defeating Ruckus will net you the Beskar style armor piece for The Mandalorian's right arm.
This is one of the most difficult quests in Fortnite. Here is the guide.
Where To Find Ruckus
Finding Ruckus is a quest in itself. Ruckus is located in Hydro 16, a large building southeast of Slurpy Swamp. This building is near the dam. Ruckus patrols the interior of the building. At certain times, he also patrols outside the building.
How To Defeat Ruckus
Prepare to bring with you good weapons and health packs. Shield potions are also very helpful. Ruckus will only go aggro if he sees you. So, take some headshots from a distance first, to reduce his HP significantly.
By itself, defeating Ruckus is a difficult quest to complete, what makes this quest incredibly frustrating is that you will have to compete with other players trying to complete the quest for that Mandalorian outfit. Anyone who wants to defeat Ruckus will definitely kill you first.
Aside from the Beskar piece, Ruckus drops amazing loot that can be used in-game.