First off you have to unlock CLU'S double barrel. You won't be able to significantly damage him without it. Once you unlock it I would do a few things to beat him:

1.Don't try going after him, go after all the powerups on the gird, you will have an extra power to beat him with and he won't be able to pick those powerups and use them on you!

2.Hide around corners and take shots at CLU. Some corners on the grid you can't shoot through so they give you minimal protection!

3.Shoot across bridges! some bridges are at the same height as others and you can shoot straight across and hit your opponent. the best way to do this is line up with the icon showing where CLU is off-screen and fire at will! CLU usually won't use this technique against you, but be careful if he decides to change his mind.

4.Stick and move! Once you hit CLU a few times get out of there! Don't ever stay in the same place for too long for CLU will hunt you down and easily pick you off! Find a powerup or health pak quick. Rinse and repeat!

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You have to check all the challenges in each round and see what the rewards are. You have to place first in these challenges to unlock pieces of it. I think there are 3 pieces to completely unlock the double barrel. After you unlock all of the pieces then you go into customize and change your tank turret to the double barrel.

Your welcome fellow program. :-)