Ashen

This slightly stronger enemy is found to the NW area near Vagrant's Rest. This area is unmarked, but is a "Brutish" area once you enter it. You'll find it just north of the "Restless Knoll" as this small self-contained section. You enter it easily by running and jumping the top area of the waterfall. For further clarification the largest body of river has this pinched area that connects to the Vagrant's Rest core, that's where you hop across.

Fairly easy boss, there are a few secondary enemies to deal with too. It might be best to revisit this later for a quicker battle. I'd say take advantage of the side cliff area to the right as that might split the group up since the brute directly attacks you.

Here's a map of it from Gamerheadquarters

In this video we show you how to get to the Brute Island Achievement: