Before this battle, go to the upgrade menu and make sure you have the melee attack volley. Then, fight. Depending on your health level, you may or may not want to use the power up. At the moment, you can easily jump over Venom, confuse him, and chip away at his health. Keep doing this until his black suit begins to "glow". Now when you jump over him he will run away. This is a good opportunity to use your new attack and chip at him, occasionally giving some hits. Keep hitting him until his "glow" gets bigger and eventually Harry will tell you to knock Venom into the air. He'll fall back to the ground same as before, easily confused. Repeat this cycle until his health is really low and you can finish him off.