You should load up the mission 'Winter Contingency'.
The next step is to look out for the ostrich type creatures in this area, they are quite fast so if you are not confident enough shooting fast moving objects i would select the 'MA37 Assault Rifle' and just spray in their direction.
After landing and walking past the initial checkpoint, 2 of them should spawn ahead of you as you are walking to the edge of the cliff. Get ready to shoot :)
The next lot that can be found are after crossing the river from the first firefight, I found if you go to the right of the river and stand at the start of the bridge, switch to your magnum and zoom in on them in the distance. This way they don't run off from the sight of you and they are easier to kill.
After the second firefight, jump down the cliff side and be prepared for the next lot of Moa. I counted 4 or 5 here so this should get you the achievement. They run quite fast away from you so if you move slowly until you see them and use the same method as before with your magnum.
Hope this Helps :)