Faster to do when you escape from the bank - the part, when you join up with Lynch and wait for the getaway car.
You will see many cop cars outside and in about 5 seconds after you approach the bank entrance 4 cops will happily run and stay in perfect line in front of the bank door with the 5th one sitting by the car (5 cops in line). Now just throw the grenade in the middle - boom, easy money. It could take a few tries, since grenades bounce very randomly.
For additional details watch this:
Video by Reabo (thank you)