Finish Act 3 by defeating Iriki
There are two separate bonus items which you can choose between to help with this battle. One of these is for helping free the serpents you meet in the pits (a mirror which I chose to use), the other is a gem from the temple at the back right of the map. You can collect both but only equip one at a time.
If you do all optional quests then you will find the items with no problem. And if you have made it this far the boss fight itself shouldn't be an issue either.