Unlock the Real Mega Buster, two bottles of wine, have at least one mini-chainsaw, the Criminal Biology, Engineering, and Entertainment books, and reach Level 50. You will be able to kill anything with one or two hits. Note: To get the Unbreakable achievement you only need to play Overtime mode and not get caught by the Spec Ops (i.e. do not lose all your health). If you started playing from 72 Hours mode, being captured by the Cult Members also affects this achievement.