To get the achievement, I was looking for Cubzero on the highest mountain (in Fresia Pass) for about 4 hours with no luck. I was, however, running into its 2nd and 3rd evolutions, so I thought I was in the right patch of grass.

Turns out, I wasn't. There are two patches of grass that are identical in shape; one in Fresia Pass (correct), and one in Frostpeak (incorrect). And in the wrong patch, you will still find Aroara and Bearealis, for some ridiculous reason. In the correct patch, you will find Cubzero and Bearealis, but not Aroara.

Are you in the wrong patch of grass?

The incorrect patch of grass looks like this:


Notice the item chest is to the left of the grass.


If you're to the East of this long ice path, you're at the wrong patch of grass for Cubzero. (But you will still find its 2nd and 3rd evolutions.) Take this path West and you will be closer to the correct patch of grass.


The correct patch of grass looks like this:


Here, the item chest is to the right of the grass. In this patch, you will find Cubzero and Bearealis.