These things are fantastic! Though they will not save you in every battle they are again a nice buffer between your opponents damage and your HP which you want to protect with litterally your life.

However, these things can be an absolute nightmare at times to level up. You can find these upgrade materials in the Diamond Shop as well as through all of your major bosses, Events and Arena but they are all expensive in some way or another or the drop rate is so low on the higher tiers that you will really want to think twice about selling it because of it's high sell value. These can be leveled up way past SSS ranking and will get stronger and provide more bonuses the higher you get. They will also recharge over a certain amount of time to allow you to re-engage in any fight you want to get into.