This would be the timed battle, i am guessing?
Once Venom finishes his attack sequence hit him with a combo by hitting the square button multiple times- best one to use is square, square, square, triangle. Once that is done get away to avoid his counter attack. It's advised NOT to jump and attatch a web line to him- that's just asking to be pummelled.
When about 1/4 of his health is gone venom will start pulling himself along the floor with his webbing- do not touch him when this happens. If your adrenaline attack meter is full use that by hitting L1 and circle- this takes a hefty chunk off.
When he gets to the point of having tentacles coming off you'll hear Goblin say "Knock him up into the air and I'll do the rest!" Hit the triangle button when the prompt comes on screen- Spidey will kick venom into the air for harry to slash him a few times and knock him back down. On landing run over and get a few punches in. then jump away. Repeat this a few times and the battle is done.