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Your weapons will do damage, just not very much. Stay on the ground and keep the ruins of the cannon between you and the boss. Just keep running in the same direction as he does (both of you running clockwise or both running counter-clockwise) to keep him from hitting you. When your ready to hit him just let him come into the clearing and blast him then get the rubble between the two of you before he can shoot again. Rinse and repeat. It takes a while but if you have good timing you'll take him down and take little or no damage in the process.