Two Worlds II

Fairly straightforward - simply equip your bow, press Y to select Multi Arrows (a little arrow should pop up over the hotkey icon), hold RT to charge up your shot and release RT to fire. You don't need to target multiple creatures - lone enemies count towards your total as well.
Let your enemies get pretty close before firing, as more of your arrows will find their mark, doing more damage. Just remember that it takes a little while for the Multi Arrow skill to recharge - normally I just sprinted away from the enemy if my first salvo didn't kill them, as they usually lose interest and wander back to their starting point. (AI is not a strong-point of TWII)

Other tips...
Best place to rack up lots of kills is the savannah - most of the animals go down pretty easily.
Advance a few levels and put some skill points into metallurgy (so you can upgrade your bow) and multi-arrow (fire more arrows & do more damage).
Use your crafting skills to upgrade your bow and add a couple of gem slots. Add an accuracy gem and an elemental damage one if you can find one.
By the time you reach Level 10, you should be able to take the weaker savannah animals out with one salvo and after that it's just a case of grinding out the 50 kills.

This one requires the ability Multi Arrows. Find several enemies near each other (2 at a time will do) and equip a bow. Press Y to equipe the skill Multi Arrow. Zoom in by pressing LT, then aim at the first foe and press LT again. A red X should appear. If it's blue, it means it didn't work. Aim at the second and press LT again to mark it too. Once both are marked with a red X, shoot with RT. To count as a multi arrow kill, the enemy (at least one, but better both at the same time) must die with this strike. You can lower their health first, but it makes it harder since they will attack you.

To make it easier, better aim at enemies that are near each other and from a higher place. There are tons of places with high rocks that you can climb by running and jumping on and which enemies can't climb. From there, you can take your time and kill all surrounding enemies, repeating the process if they don't die on the first strike.

All enemies are accountable for that achievement, meaning every single foe marked by a red icon on your map. Friendly animals such as chicken, horses and birds do not count.

You can keep track of your multi arrows kills by pressing B (when no weapon is equiped).