When using a sword, your Slash ability will reset if it scores a critical hit.
You can boost your critical hit chance in many ways. To name some of the more easily obtainable ones, you can use Strength destiny cards. Using the Berserk Aura demon spell will give you a 20% increase for it's duration. The vigilante outfit gives you a 10% increase. You can also guarantee that your first hit will crit by attacking an enemy with your sword 3 times before starting to use Slash, as this will apply vulnerability which makes the next hit the enemy takes be a guaranteed critical hit.
It doesn't matter if the enemy is immune to critical damage as the ability will still reset. Also make sure you have enough armor penetration or the enemy has low enough armor in general so that you wont get a glancing blow by chance.
If you have either DLC, a very easy way to get this is to attack the back of a scorpion as it guarantees a critical strike. If you press the button too fast, you will push the enemy too far away from you. Either attack in a rhythm or pin one against a wall.