With multiple players in a match, enemies will lock on and target party members besides yourself. When you see an enemy or boss targeting a teammate, an hour glass will appear above their head signifying they are casting or attacking; charge to the enemy and interrupt them by using shield break (press X ). Upon a successful interruption you will see "Interruption" above the enemies' head, whether it be 'fast' or 'late'.
A good spot to grind this is at the end of the Alkazaba Mines. Just make sure the enemy is targeting your co-op partner and not you.. as long as you don't turn the wheels, the enemies will continually spawn.