Hi TheOneWolf,
The proper way would be to make a custom collision in whatever modeling software you are using.
You also have an option of using the polygons themselves as collision but this is much more taxing than the option above (especially on something as large as a landscape). To do this, open the asset in the Static Mesh Editor. Under the Details panel > Static Mesh Settings change the Collision Complexity to Use Complex Collision as Simple.
I hope this helps.
Thanks, TJ