it took me all of 10 minutes at the most to beat those levels, I think. When you come to each of the two rocket guys, all you have to do is stop the cart around the corner, where he can't see you and get off on one side. Run around the corner on the far side, drawing his fire. When the rockets are close to hitting you, jump the tracks to the other side of the rails and run like hell.
When the rockets explode, crouch and walk. He can't hit you when you're crouched. Just pull up your six-shooter/crossbow and hit him in the head/shin. Go back and get the cart.
Then move on.
There are two of them, and you can take them bot out the same way.