Assassins creed brotherhood can i manually aim at targets?

i have been having alot of trouble with the pistol and crossbow because the game selects who i aim at. today i was trying to aim the pistol at a brute with an agile standing near him, both in direct line of sight. the brute was closer but every time i aimed at him with the pistol it would automatically switch my aim to the agile standing a couple of meters to his right. how do i change to aiming system to stay locked on to the target i select?