Let me tell ya, its not easy to score, those 'skill shots' are very easy to block even if you shoot 6 at full power.
I've been playing online mostly and moment ago I tried extremely difficult mode (which I lost both games - it's very hard)
Online, its very challenging if your opponent is good (obviously, usually a normal game goes for 2 or 3 goals at most when both players do well).
In Single player
Normal mode ( very easy)
Extreme mode (impossible)
Also a game can last from 3 minutes to 20mins (if your both tied, it goes into sudden death, whoever scores wins).