Brain Freeze

The Brain Freeze is one of the Starting Ruins Colossal Titans you need to defeat and also was featured in the game's demo. It's subtitle says Brain of the Guardian

Tips for Beating
Shoot an Arrow from the entrance position, then roll diagonally upwards to dodge the first charge. Roll to the center once and retrieve the arrow. Align yourself on the opposite side of the fireplace compared to the boss and when he activates the plate, shoot through the flame to expose the frozen Brain. Then prepare an arrow while the animation plays, and shoot the brain as it first slams down. (Credit: u/LordJellyfish)

Another way to defrost the brain is to shoot your arrow right on the fireplace(A fully-charged shot fire through entire arena for your reference). While dodging, the titan will trigger the fireplace and the fire on the arrow will stay for like 3 seconds. All you have to do do is keep dodging and kite the titan land on the burning arrow