After beating the final boss for the first time you'll gain access to Hexen Isle and the Calix dungeons. Complete Calix I-VIII and you gain access to a boss. To find the boss, go to the dragon bones discovery point and check it and you'll be in a battle.
Only this battle has to be completed on normal or above. Everything else can be done on simple and this achievement will still unlock.
Be aware, winning this battle will boost the power of the final boss, so make sure you got the achievement for beating the boss on chaos already before doing this.