User blog comment:Blademaster00/Official Amatsumagatsuchi Talk Page/@comment-189.120.135.61-20110202004744/@comment-91.179.175.88-20110203102221

The chest can't be broken, it's a mistake. Like you said, chest glows orange/gold when Amatsu is in rage mode, but there isn't any break part on the chest unlike actual Leviathans in Tri (Lagiacrus and Agnaktor). However, dealing enough damage to Amatsu's chest will make him "fall" and give a good opening to attack the head or the back fins.