To get the Tranquilizer Sniper, you must defeat The End by depleting his stamina, one easier way to do this is to follow his footsteps with the Thermal Goggles, he must likely won't espect you to be behind him but he will hear you if you just go running. If he spots you from far, don't try shooting from there, instead try gessing where he'll go next. Once you defeat him, his sniper will be at the center of the north part of the forest (Sokroveno North).