Great idea!
I'd recommend following the same formula from the old solo artist challenge, and have different tiers of achievements.
For example it wasn't an easy fight to solo Dhamph at 35, but compared to most of the mobs on your list it's easier.
For the big Dino guy you have to train a Spectre or Cyclops to the island and you just make sure your pet never outaggros you, which isn't that hard spamming chains and the pet clearing its own aggro every 18 secs helps too
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cheers,
Astuce