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)
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.