The Skeleton Sniper is an uncommon post-PlanteraHardmodeDungeonenemy that attacks from long range. A Skeleton Sniper approaches a targeted player slowly and, as soon as it has a clear line-of-sight to the player, pauses briefly before firing a single shot. This shot can be fired from offscreen, and deals enough damage to kill a player with maximum health in three hits (two in Expert mode).
These enemies spawn relatively infrequently and are considered by the game to be rare monsters, causing them to appear on the Lifeform Analyzer.
The Skeleton Sniper spawns most often in areas of the Dungeon backed by Slab Walls, along with Rusty Armored Bones and Ragged Casters. Removing these naturally occurring back walls will prevent these enemies from spawning, but they cannot be replaced to spawn these enemies elsewhere. These areas can further be identified by the presence of Ragged Brotherhood Sigils and Rusted Company Standards, but these banners are purely indicative and do not influence spawning.
The Sniper must pause again after being hit, so rapid-firing long-range weapons at it will prevent it from attacking.
Weapons with good knockback are not advisable for this enemy, because knocking it back far enough will make it pause instantly when on solid ground and shoot the player with ease.
Trivia
The Skeleton Sniper appears to be using a different gun than the Sniper Rifle it drops.
He also appears to be using a German World War II-style helmet.