Here is an image of the best aggro spike fight back spot in a Hospital, located on the top 4th floor in a computer room. This spot is only used if you don't have a Grenade Launcher and a random aggro spike occurred. Make sure you stand behind the desk in front of the door entrance. Rifles, pistols, Machine Guns and Shotguns work best at this particular spot. But I recommend using pistols and rifles for a cheap grind and to save ammo. The Aggro spike will go for around 5 minutes.