To obtain a Cleansing Heart, you need to kill the Boss Oceanid otherwise known as Rhodeia of Loch on any World Level. Bosses are automatically Lvl 36 even at World Level 0, so it is recommended to have your Characters at roughly the same level as the Elite boss or higher.
To get the rewards for defeating this Boss, you must spend 40 Original Resin. Though if you have Condensed Resin stored, you can use this and get double the rewards.
To find Oceanid, you can open up your Adventure Handbook, head to the Enemies Tab, locate Oceanid, then click on the Navigate button. The game will show you where the Boss is on your map.
Or you can head to the location shown below and cut out the middle man.
Just click on the image below to enlarge it.
- Type: Elemental Stone
Cleansing Heart is used for the following Character Ascensions.