Phantasy Star Online 2 New Genesis

While this enemy will likely be available through a future story mission for more reliable access, as of the 6/14/2023 update it can now be found as a random boss in the "[Rank 5] Enemy Level 70" version of Mt. Magnus in Aelio Region. After extensive grinding it does not seem to appear in either Vanford Laboratory Ruins or Resol Forest. Killing variants (Silver, Golden, Malevolent, etc) will count, as I killed a Silver Herzig as a PSE Climax boss to earn my achievement.

Herzig is just the family name for "Starless" Daiytl Sword enemies. Those are the very tall bipedal enemies that usually have a large yellow weak point on the back of their neck, as seen below. Note that Daiytl Sword itself is still a possible boss spawn in Rank 5 Vanford Laboratory Ruins, but this will not count.

"Starless" enemies presently only spawn in Rank 5 Mt. Magnus, Rank 5 Vanford Laboratory Ruins and Rank 4 Resol Forest. These higher rank combat sectors are only available after earning a Battle Power rating of 3119 or greater via the Main Task New Raiding Missions. This task itself, per user Ixtab Lullaby in the comments below, only unlocks after clearing the Main Task appropriately titled "Starless", A New Enemy. If you're checking progress via the Quest Counter Admin, this will be labeled as [Scene] Chapter 6: As One Heart.