If for some reason you find yourself missing a few of these after playing the campaign, load up checkpoint Chapter VI: Defend Iacon: The Broken Lift. Play as Optiumus Prime.
The first enemy you face is a brute. Use Prime's boost ability (LB) to get behind the brute and shoot the box on his back. It takes several hits to destroy them.
You only get credit for brutes that you deal the final blow to. Once you've beat him, restart the checkpoint and repeat until you've got your five.