That amounts to 90 separate bosses that all need to be defeated individually!
Fortunately, these achievements are not required to earn the Preying For Midnight meta, though it is still a huge inconvenience for completionists. For example, completing Prey: Lieutenant Blazewing (Nightmare) does not give credit for defeating the same boss on Normal or Heroic, which is very unusual for achievements of this nature.
Four random targets are available on each difficulty each week, and this is still the most rewarding way of progressing since they give better rewards, though once you reach Rank 10 on the Prey: Season 1 renown track, Prey: Preferential Killing allows players to fill in the missing gaps. As of May 12th hotfixes, these custom hunts are now infinitely repeatable, although you're still in for quite a long grind to finish not one, but all three achievements separately.
Random Prey hunts used to be the preferred strategy, though the inability to target specific bosses could make it an extraordinarily frustrating process.
