sprint/run 90% of the time (you can stop if you have enough distance). some bosses will require you to hold out your melee weapon to be able to loop them without getting hit. face the way you are running! only turn around to shoot if you have enough distance from the zombies, or the boss has stopped running to do an attack. the bigger the area you are looping in the better.

and make sure any teammates stay away or they will draw aggro from the zombies/boss.

don't try to melee an entire hoard at once, you will die.