actually, it's easy in a way. all you gotta do is count how many enemies you defeat at first, say 22-25, then speed to the end of the trench and destroy those gun turrets/big gun turrets. they will boost your enemy count to at least 30,which is needed for gold, and there you are! I did that and got a bronze.
Just believe this and nothing will happen, as you think
I will win either way! hehehehehehe............