In the process of running through the Arena, you should have noticed that you will see when an opponent is charging for a spell or executing a skill. These are your moments to perform a Counterattack, otherwise known as a Hangeki. The difference between this and a Rengeki is the noticeable text that displays as well as it only being obtainable within the Arena setting. You can track this via the Book of 1000 that corresponds to Arena matches.
You can acquire anywhere from 4-20 counterattacks by running through Survival Battle (five battles) and just waiting for the enemy to charge their skills. It is best used against Harvest Clerics or Shadow Warlocks, though you can do this on melee too.