You need to beat the game first so you can unlock Hard difficulty. Try the mission when you have to help Leonhardt and some other NEXT needs help destroying 4 other NEXTs. Accept the mission and you will be informed the whole team is already wiped out. If you finish this mission (at the four to one odds), you will earn two parts, one of which will be Leonhardt's back piece. Recommend is using a NEXT with good fuel and resorting to laser rifles.