The Club, Bow, and Spear all have achievements for hitting 100 kill count. Depending on your personal preference, you'll pick up at least two achievements for the weapons without really trying.
TIP - The Spears are quite heavy, so remember to aim high. Just above the head usually works.
TIP - Go collect your spears afterwards! You won't recover them by searching the bodies, you'll need to walk over them to pick them back up. Crafting them is resource intensive, it'll take you forever if you have to craft every time you throw a spear.
TIP - I believe that your takedowns will count towards the kill count. You can see your selected weapon being used for the takedown. If I or another player confirm this, I'll update the solution.