You will have to kill 7 Moa. There are an ostrich-like bird. There are a lot throughout the level (There are more than 7).
They're easy to kill, but they are fast. Just simply kill 7 during the level and the achievement will pop.
Reloading checkpoint for killing 7 at the beginning of the level won´t work. You need to kill 7 during the level. At the beginning there are 2 Moa.