Because of how he fights there is a easy tactic to defeat him if you are having hard time with Voss.

Whip

Voss has your whip and will use it on you whenever you try to walk away from him.

• When he's pulling you with the whip, your health regeneration fills one bar.
• After he pulls you close to him, he becomes vulnerable to your melee attacks for one second.

With this two things in mind you can keep walking away from him to regenerate health and attack him for three punches when you are close to him after he uses his whip on you.

Just continue doing this until he's defeated.