There are a total of 15 Unique Enemy Quests that you can accept from the Unique Enemy Squadron, here's the full list:

- Quest: GGO'S Next Top Bot lv.5 08:00 

  Location: Remnant Wasteland, Floating Tower

  Enemy: Elite Automaton lv.80

- Quest: Metal Menaces lv.15 01:42

  Location: Remnant Wasteland, Control Facility

  Enemy: Metal Ahriman lv.90, Assault Drone lv.81

- Quest: Irrational fo Rations lv.23 04:33

  Location: Remnant Wasteland, south of Extinct Civilization City

  Enemy: Wanderer Eater lv.98, Lurker Insect lv.84

- Quest: King Of The Wastes lv.16 06:51

  Location: Remnant Wasteland, Extinct Civilization Research Lab.

  Enemy: Waste Gang lv.91

- Quest: A Shell Of A Person lv.28 08:16

  Location: Solitary Sands, Ship Graveyard

  Enemy: Heartless Puppet lv.98

- Quest: Subterranean Rogue lv.30 09:55

  Location: Solitary Sands, Underground Labyrinth

  Enemy: Underworld Rogue lv.105

- Quest: One-Hit Kill lv.31 12:57

  Location: The Old South, East of City Ruins East Entrance

  Enemy: Solo Stinger lv.106, Empress Wasp lv.101, Emperor Wasp lv.100


- Quest: Wiped Clean lv.33 14:14

  Location: The Old South, Subway B

  Enemy: Omni Eater lv.109, Double Artillery lv.016


- Quest: Cursed Crystal lv.45 18:53

  Location: The Old South, Subway B

  Enemy: Cursed Rune lv.110


- Quest: Into The Labyrinth lv.43 22:02

  Location: The Old South, Abandoned Road

  Enemy: Ruins Gate lv.118, Observer lv.116


- Quest: The Worst Travel Agent lv.63 28:25

  Location: The Forgotten Woods, North-West of SBC Flugel Gate

  Enemy: Abyss Guide lv.139, Hooligan lv.115


- Quest: Head Hunting Worm lv.53 44:03

  Location: Solitary Sands, East of Worm Nest

  Enemy: Neck Biter lv.128, Caterdozer lv.120


- Quest: Honeycombed lv.68 49:23

  Location: Solitary Sands, North of Industrial Zone

  Enemy: Honeycomb lv.142


- Quest: Following Order lv.58 55:03

  Location: Solitary Sands, Underground Weapon Plant

  Enemy: Genocider lv.132 

- Quest: Movers and Shakers lv.66 56:58

  Location: SBC Glocken Underground Ruins

  Enemy:Heartshaker lv.141


After that you'll unlock the trophy "Bounty Hunter".

Check the video to see the exact locations and some tips to beat the enemies: