Getting into a scuffle with a wolf, wounding it and having it bleed out somewhere counts as a kill. I'm also pretty sure putting up a rabbit trap, even if all you find is a half-way busted trap, also counts as a kill. The statistics screen seems to point to these as kills. I have yet to confirm if fishing voids the achievement (which would make it impossible on anything but easy.)
Meaning, this achievement is best done as passive as possible. Pretty hard to do with higher difficulties. I have yet to manage not getting into a wolf fight on stalker or interloper, as the statistics show I end up in a fight every other day =)