defeating an attacker is something completely diffrent from defeating an wel entrencht defender being able to do the first doesn't nessecaraly mean you will be able to do the latter especialy if the terrain favors the defender as much as in norway.
Have you read up on the fighting in Narvik? It happened roughly like this;
1. Germans enter the Fjord, sink the two coastal defence ships and land, shattering a Norweigan battalion in the process.
2. Norweigans rally and keep fighting.
3. Allied ships sink the German destroyers.
4. Allies land troops.
5. The Norweigans are the attack part of the allied force, outflanking, infiltrating and pushing the Germans back (retaking Narvik) to the point where Dietl was one or two days from ordering his troops into Sweden to be interned.
The Norweigans knew their terrain and used it to their advantage, even when on the attack. The Greeks were capable of doing the same against the Italians winter 1940.