Easiest method I found was when you unlock the fence trap outside of the tower doors. Once you can use these, sleep until night, find some Volatiles lurking around the immediate area, have them chase you to the northern entryway, and use the fence trap on them. The trap won't kill them, but it will do decent damage to the, stun them for a very brief moment, which leaves enough time to hit them into the spiked barrel next to the door. Rinse and repeat this and you'll rank up in no time.