Play the game on the highest difficulty level. It may be frustrating or seem impossible, but it will continually force you to get better and better.
Also, you've got to determine your weaknesses and figure out how he manages to beat you. When we used to play Turok 2, some of my buddies got so good at it, they would know where I would spawn and kill me before I could even grab a weapon, so I got very good at headshots with the crossbow lol