I've been playing around a bit and i think I've found the best soultion.
Mode: Total Chaos
Weapon: Nano rifle
Building Collapse: High
The backpack is the most important thing here. When you start start blasting randomly at the nearest building to clear a door into it. Walk through the building while fireing using the most indirect route, while your doing this make sure your backpack is on. Only stay in each building for the duration of one firing of the backpack otherwise you wont get through them all.
Two things to note the buildings wont always collapse right away but usally do before you run out of time, also it might fall on you or barrels might exploded (killing you) don't worry you should still have enough time. Even on my worse run i was getting 120K best coming out at 178k it's a little luck but requires no skill.