There are only about 7 guards and 2 to 4 civilians on Normal difficulty. Go through the heist as normal until the exit is unlocked. After that, use any remaining pagers to eliminate the guards farthest from the exit, then, use a loud weapon to alert the remaining guards and place an ECM that blocks pagers. This will this will show you where the remaining 3 or so guards are so you can kill them as quickly as possible before completing the heist.