Good old glitch...
The video has all details, it hasn't been patched and I'm 99.9% sure it never will be because you know, effort.
Only 2 things to add;
1: Spitters have a decent attack range so back off into the trees a bit.
2: It's pretty damn easy to get both dudes in there so if you want to speed it up, it's well worth getting them both in the safe zone.
Just rubber band the controller and go and watch a great movie such as "The Room", "Revenge of the Sith" or "Paul Blart Mall Cop".