For this achievement you will need a minimum of
4000 points. Also, for this I highly recommend that you use the Elixir called
Shields up as it allows you to spawn a shield without having to build it (thanks to ihaze reaper for reminding me of the Shields up Elixir).
From spawn (near the Strife wall buy) you will want to make your way to the
Amphitheater (Artifact Room) to retrieve the artifact. Once you have done that make your way back to the
Marketplace and buy the door that leads to the
Gymnasium Bathhouse, go through the Gymnasium Bathhouse and you will be in the
Intersection of Treasuries.
Once in the Intersection of Treasuries you will need to look for the
Golden Bridle. It can either spawn on the small pillar on your
left side as you enter the Intersection of Treasuries, or it could spawn in the
Stoa of the Anthenians area on your
right side on a small pillar. Once you have picked the Golden Bridle up from one of those two locations, a
Gegenees will spawn near where you are.
Once the Gegenees has spawned use your Shields up Elixir and keep on using the harpoon attack against him. Since it will be a low round he will go down relatively quickly.
Overall this achievement is pretty easy to do I think.
If you are still struggling, feel free to use this as a reference.
Hope this helped, thanks.