Countering is the most important thing whilst fighting Deathstroke.

All you need to to is to counter him when the blue halo appears above his head, then hit him as much as possible. He will try to counter you with a small interactive scene. Wait for the prompts to counter him during this scene to counter his countering. Once you are through the QTE hit him as much as possible.

Deathstroke will try and use his surroundings to damage you. This activates a QTE where you only have a fraction of a second to press counter.

Deathstroke will then jump onto you. Repeatedly hit the counter button, until it prompts you to repeatedly hit him over and over again. This is how you will deal the most damage to him during this fight.