Instructions
Complete "Dead Rising" once. By default, the only mode you get is the 72-hour mode, and you need to complete it to unlock Overtime mode.
Select "Overtime" when starting a new game.
Complete the case files. Like the base game, Overtime has missions you must complete within a certain time frame. These are simple to get through; the real danger lies with the soldiers trying to defeat you.
Replenish your health frequently. Overtime mode consists of 24 in-game hours during which the security forces occupy the mall. Dying in this mode is different than it is in the base game. The special forces knock you out and tie you to a chair, invalidating the achievement. Keeping your health up prevents this from happening.
Avoid groups of special force soldiers. Much like the base game's psychopaths, security officers wield machine guns, and a group of soldiers can turn your character into Swiss cheese in a matter of seconds. However, the soldiers are much less lethal when attacked by themselves or in groups of two or three.
Attack the soldiers using melee combat. Because soldiers are heavily armored, using a bladed melee weapon, such as a knife or a katana, works best in your favor. For example, using a katana when you are level 50 defeats a soldier in two attacks. Alternatively, use hand-to-hand combat moves, such as the zombie throw, which defeats soldiers with one attack.
Survive the full 24 hours without being knocked out. Following the previous steps will ensure that you do not succumb to the soldiers' efforts to defeat you, and when you reach the heliport at noon, you will unlock the true ending of the game, earning you the Unbreakable achievement.