Mallcrasher is probably Payday 2's most straightforward job: you're tasked with causing a set amount of monetary damage to the mall. The amount differs depending on difficulty, but the strategy is the same.
First of all, casing the area is not necessary - your destruction will alert everyone, pretty much guaranteed. That being said, you should wait to put on your mask. Make your way to the top floor, and split into pairs, one on either side of the mall. Ready yourselves for destruction, and have at it.
You may be confused as to what counts toward the total damage for this job. All glass panels count, as well as anything within the stores. Shoot the lowest shelf on racks to spill the whole thing, saving time and ammo. It's also wise to purchase the Gas Can asset before starting the mission. Planting this in a clothing store, the shooting it will burn the store from within, causing massive damage.
Finally, make sure to equip an Ammo Bag for this job. As you'll literally be dumping ammo into everything around you, you are going to run out.