Gloomhaven Rule Book, Page 29, Monster Focus wrote:
The first priority of a monster is to focus on the enemy closest to it.
The archer will focus on the Brute.
Gloomhaven Rule Book, Page 30, Monster Movement wrote:
If the monster is performing a ranged attack, it will only move until it is within range to perform its best possible attack. A monster will also move away from its focused enemy until it can perform the ranged attack without Disadvantage.
It will move one space away from the Brute to perform an attack against him without Disadvantage.