You can do this by using a <code>while</code> loop and damage every single enemy in the fire's location every second.
obj/fire New() ..() src.FireDamage() // when the fire is created, call the FireDamage() proc to start the damaging method
proc/FireDamage() spawn while(1) // loop forever, but allow the proc to end for(var/mob/NPC/M in src.loc) M.TakeDamage(10) sleep(10) // every 10 ticks (usually one second) damage every single NPC in the fire's location
~~> Unknown Person