This is being written from the perspective of a ranger so bear that in mind if you play one of the other classes.
Probably the hardest achievement in the game, requiring both luck and skill to pull off. I recommend beating the boss a few times to learn his attack pattern and practice the dodge rolls you'll need to pull this off (and if you get lucky find the blueprint for the titanium weapon of your choosing). Also waiting until level 30 will make this easier but is not necessary. Ideally you want to be able to kill the boss in 2-3 rounds.
I did this as a level 26 ranger, stats, skills, and gear as follows:
-75 agility / 110 dexterity (other stats largely don't matter for this achievement)
-Bow specialization / Sentry stance / Dodge chance / Dodger / Sharpshooter (In the face of evil may be better in this slot) / Combo swing or Exploit weakness
-Assassin's hood, tunic, and gauntlets, Bandit pants (for the double jump), Marksman cape, Dark ring of eternity, and Sunna the golden longbow
Other items that may help but aren't necessary:
-Star Flask +20% Strength/Agility/Intelligence for two minutes
-Ranged Damage Flask +30% Ranged damage for two minutes
-Swiftness Potion 1/2 - +40%/+80% movement speed for two minutes
Now on to the actual fight. Make sure your weapon is fully repaired before starting, you don't want to have to repair mid fight. The idea is to destroy the three crystals scattered around the arena which causes the boss to revert to his icon form and makes him susceptible to damage, all while dodging his attack pattern. His pattern is always the difficult to dodge spinning blade attack, followed by 3 of the highly damaging, but easy to dodge dark orbs.
The spinning blade requires some luck and skill to completely dodge. What you want to do is when the boss goes to do his spin attack, have a crystal targeted (which gives you the ability to dodge roll). As he approaches you, mash
to roll perpendicular to his path of travel until you are completely clear of him. As soon as you are past him start shooting the crystals while keeping an eye on the boss to dodge the orb attacks. Destroy all 3 crystals and he will start glowing (he is vulnerable to damage here but be cautious of him getting one last orb attack off) and eventually shrink down into his icon form. Lay as much damage as you can on him before he returns to his invincible form and spawns more crystals, repeating the pattern as before.
As I said above, this requires some luck and will probably take a few tries but dodge rolling was the most reliable way I found to consistently dodge the spinning blade attack (with a few double jumps thrown in when I got into trouble). Fortunately the boss drops decent xp, titanium bars/ore, astral hearts, and blueprints for end game gear (along with a piece of the rabbit set) so farming him until you get this achievement will be beneficial.
Edit: Managed to do it again and actually remembered to record it this time.
Good luck!