Runners are people that just became infected. They still have the ability to run, jump, etc. There is an easy way to get them, regardless of where you are. Lead them to a bigger vehicle, such as a van of some sort. It will take awhile for Runners to get on top of it. Use that opportunity to take them out.
The monsters that come out at night are Volatiles. They are very fast and deadly. However, their biggest fear is electricity. If you have a weapon that has the "electrocutioner" or some other form of electricity on a sword of some type, us it. A few good swings will bring them down. Also, these monsters often carry a lot of money; search them after you take them out.
Find a burning zombie and kick or throw it on a spike trap.
Jump up on a van or bus on the overpasses or the big bridge. Lure a crowd with firecrackers. When about twenty of them have been attracted, throw a Molotov cocktail at them.
Use the mod that is awarded for completing the Gunsmith side quest at the Old Town.
This can be done in single player mode if you have the Cuisine & Cargo DLC. Quarantine Zones are counted as coop competitions, even when completed in single player mode. With the DLC there are ten Quarantine Zones in total. Complete all of them to earn "Harran Athletics".
A wrench, the Vault skill, and the Drop Attack skill are required. Find two zombies then equip your wrench. Line up the zombies, then run and vault over the first zombie. Drop attack the second zombie with your wrench.