So shotgun 1 enemy THEN punch them all to death 
Though if you kill quickly with a shotgun (easy to stay accurate) you'd get more from the below par time thing so punching probably isn't worth bothering with. Besides, you get plenty of Shotgun ammo throughout.

Though if you kill quickly with a shotgun (easy to stay accurate) you'd get more from the below par time thing so punching probably isn't worth bothering with. Besides, you get plenty of Shotgun ammo throughout.