When the Mystic Armor is equipped, you lose Rupees instead of taking damage. Each level of the effect determines how many hearts you can avoid losing and the number of Rupees it will cost. So be careful when fighting stronger enemies and bosses. 1 Heart will cost you 40 Rupees, so even the weakest attacks (1/4 of a heart) will cost you 10 Rupees.
Lose up to 120 Rupees to save 3 Hearts.
Lose up to 240 Rupees to save 6 Hearts.
Lose up to 360 Rupees to save 9 Hearts.
Armor Of The Same Set
Where To Get Mystic Headpiece
Get From Exchanging With Bubbul Gem
Visual of Air Balloon
When you speak to Kilton near Woodland Stable in the left bottom of Eldin Canyon, then it will activate the quest. During the conversation Koltin will appear and you will be able to exchange Bubbul Gem to a Mystic Headpiece. Kilton is by the air balloon so use that as a landmark.