Guide:
Lost City of the Na-Totambu is the public dungeon of Alik’r Desert and it is several large rooms with bosses and groups of 4-5 mobs you must overcome to get to the skyshard. Entrance to this public dungeon is east of Sep’s Spine Wayshrine and is marked on the big map. Screenshots contain the exact location of the shard inside the dungeon.
Collectible is in a hole in the big central chamber of the Lost City. You can just drop down into the hole with the shard, but be careful because there is a Gargoyle guarding it. You can pick it up without alerting the Gargoyle and effectively solo picking it up. Otherwise bring some friends along because the mobs are tough for soloing.