Your new opponent is the Bone Giant, which has three basic types of attacks. Arm attacks are easy to notice, but it doesn't mean dodging them is easy as well.
I recommend paying much closer attention to the head attack of sorts (the above screen), in which the boss thrusts out his spine. The attack is quite fast and not as easily predictable as his regular ones.
Hit the Bone Giant the reliable way by pulling yourself to him with Death Grip after each unsuccessful attack. Remember not to too overdo it, though.
Due to his strong resistance, you may need to use the Reaper Form to get rid of him quicker. Keep attacking until he's eliminated.