omg what. reaching the end alone took me so many attempts, how exactly did you get such an extraordinary score??? lol
Focusing on upgrading salary early on, though not neglecting detect, then just slowly clearing out pretty much everything on the map. A lot of it is just using logic to figure out what is guaranteed not an undead. Getting detect to at least 3 can help with most cases, but detect 4 will help out near the end. Even detect 1 can get you pretty far. I tried to maximize the resources that show up, getting 2 health off of an undead with bullets will allow the next melee hit to take it out. I leveled up survival to get the campfire upgrade to allow more heals.
In the end, I did have to pass up some undead since there the one empty spot that would score them was blocked off and I didn't want to risk taking health or wasting bullets. Reaching the town doubles the score, so I actually got about 15213. Going through the map as thorough as possible did take me a few hours.