Go to the Earthrock Trilobyte level with Axl to the point where you have to start chasing the giant Mechaniloid. Then, just have Axl bombard it with his gun while your chasing him, and he'll drop TONS of red metals. Also helps to proceed slowly, cause while your standing still, he won't be going anywhere, and you can get more metals off of him. And before he gets away, jump into the spikes to use a retry chip and try again. You can get over 10,000 metals in one trip with a little luck. The trick also works with X, but it works better with Axl due to his rapid-fire ability.