Sludge Heart

The Heart of the Guardian; Sludge Heart is one of the Starting Ruins Colossal Titans you'll come across in Titan Souls.

Tips for Beating
Each shot you shoot will break the sludge into various parts, with the heart being the one that you'll want to expose. If you are able to only target the heart you can make the process a little bit simpler, rather than having all sorts of sludge pieces chasing you around. Once the heart is exposed, simply put the last arrow into it to defeat it.