Weak Brute Hunting is a strategy Brute players use to train their primary brutes. They find weak or lower level brutes to fight so as to make sure they can get the maximum of 2 experience for the fight. Fighting lower level brutes will make it easier to win in the Arena, but if you fight a brute that is too far below your level you risk only getting a maximum of 1 XP for winning.
The cutoff point for this is 2 levels. Meaning, if your brute is level 10 and won from a level 7, you'd only get 1 XP for winning. If you beat a level 8 you'd gain 2 XP.
To easily find brutes below your level you can search the appropriate rankings page on the official website.