To get "Chaos Harpy" achievement you have to join "Mirror portal" (/join mirrorportal).
Then kill that "Chaos Harpy". But it is hard to kill that monster because
it gives 300+ dot and also cannot be stunned.
You can kill it alone if you are already in level 55. But you have to use "shriekward potion"
Use it when You are under the effects of a Harpy Naga's powerful Shriek. Do NOT use it before you are affected by a Harpy Naga's shriek. Because it won't work if you do that.
You can get this potion from "Shriekward potions" shop (/join mirrorportal).
Note: Stacks up to 10.
And remember to accept "Horror Takes Flight" quest.