Back 4 Blood

*Small Aside: I am currently writing this at a McDonalds as power lines near my house were cut in half following a storm. Some information in the solution might not be 100% accurate due to not being able to double-check this either in game or not having time to cross-reference. I plan on editing this once power is restored.*

Skull Totems are found within Ridden Hives, which are an optional challenge that can be toggled to spawn on or off when creating a new campaign, however I believe the default is on. When inside the hive these totems will be on a ledge which needs to be jumped to (as far as i've seen). The Skull Totem will replace your primary weapon while you hold it and carrying it to the exit will give you one currency per token held by the party. All you need to do is hold this totem and kill any 10 Ridden with it. I did this solo so I have no idea if party members killing ridden count towards your achievement progress, but I would assume it does not.