Board Thread:Theories/@comment-27441457-20151225042055/@comment-27321453-20160607052014

Eren was only able to harden after drinking what's in that bottle. Reiner has a chink in his armor, the back of his titan's knees, Mikasa was able to slash them back in chapter 44 not to mention Eren was able to break his "armored" arm a bit too easy in that same chapter and that was before Eren knew he has the Coordinate much less harden his titan form.