To get Unbreakable, you have to finish 72-Hour Mode and Overtime Mode without being knocked out*, so save often, and if you ever get killed by a zombie, captured by the cultists or captured by the special forces, load your game from before you got knocked out. A good thing to do near then end of 72-hour mode, is to stock up on mini chainsaws(They spawn in the Space Rider in Wonderland Plaza after you defeat Adam the clown), since they kill a spec ops soldier in 2-hits, and steal a Machine Gun from a dead spec-ops soldier to take out the flying drones from a distance in Overtime Mode.
In Overtime Mode, when fighting the tank, the little laser targeting dome should take priority over the drones, or the lights above the treads, once the tanks health is depleted, the best way to kill Brock depends on Franks Level, If you only have the Jump Kick(unlocked at level 5), stay on the lower level of the tank, and use that, if you have the Double Lariat,(unlocked at level 20) jump onto the top part of the tank, and use that, before retreating to the lower part when he attacks you, if you have the Somersault Kick(unlocked at level 44), you should have no problem
*as far as I'm aware in the one playthrough, which means you can't finish 72-hour mode without getting knocked out, then replay 72-Hour mode with a higher level frank, get knocked out, then finish Overtime mode without being knocked out. It has to be consecutive.