Well, from my experience (only 3 seasons):
At times, I played against very weak (10Q+ lower), but 2 star players managed to score against 5 star defense and goalkeeper (irritating, yes). So anyone who says that formation doesn't mean a thing, haven't paid much attention to the way engine works. Those who believe that formation is everything are wrong as well, there are formations which will give you advantage over another, but there's no magical formula out there. Never put nonsense arrows, even to your front-men, it will screw your game greatly.

That being said, playing with very weak players is no good as well, they won't perform very well, lose the ball and you will get a number of counters at your gates.

If you provide more details, perhaps it would be possible to help you.