Get on the opposite platform from the Ruin. This is to have a gap between the two platforms separating you and the Ruin. The Ruin will not try to cross the bridge. The gap will prevent it from charging you. Fire the Pepper Grinder at the doll heads that stick out of its body. Throw clockwork bombs to distract that Insidious Ruin that appear during the battle. When the Colossal Ruin loses all the doll heads and starts to charge you with its flame attack, abandon the gap and face it normally. Use the Teapot Cannon on the giant doll head that appears and attack him as usual. If you try to knock him down and stun him with the gap between you, an invisible wall will prevent you from hitting.