Attack on Titan: Before the Fall Vol. 12 (manga)

Angel and Xenophon's Device, meant to help humanity combat the Titans, has always confounded those who were meant to use it. But years later, after Sharle successfully brings Angel out of his underground home, he is ready to correct its fatal flaw for good. But that's only half of the problem—the Survey Corps will need a grueling new training program to make use of it!