Of all the people and places you would choose, I really don’t see the Norsemen having any real trouble fighting the giants on your terms. I see some glorious battles in your future.
First off, if they are Vikings, they will fight like Vikings. The Vikings were very mobile armies and favored offensive tactics. Before the giants show up at the gates, they will have been engaged at least once, maybe three or four times out in the field. In the steep slopes of the fjords, using the terrain to your advantage would quickly eliminate the giant’s height advantage. Given that the giants are not too bright, this might work quite often.
The axes favored by the Norse, especially the two handed ax, could cut chainmail and split iron helms. Thick skin is not going to be that much of a problem unless it is actual stone.
Norse warriors were taught from childhood how to catch a thrown spear and return it to the sender. 358
If the spear or rock is too large to catch, simply stepping out of the way seems trivial.
The Norse also favored heavy wooden shields and very quickly closing with their opponents. If the shield blocks that one spear jab while your friend closes to close combat, it may not matter that you get knocked on your rear.
Last, given what I have read about the Norse berserkers, in a one-on-one against a giant, I do not favor the giant’s odds.
Back to the siege, the Norse were extremely adaptable to their environment. There were rarely stone fortifications in that time period as there were no Frankish navies laying siege to Scandinavia. When the Norse men moved into Scotland or were invited into the northern Frankish territories (becoming known as the Normans) they adapted to the needs of the region and built stone keeps all over the landscape. If your settlements need stone walls, they will build stone walls. If they need ballista, catapults, or trebuchet they will build ballista, catapults, or trebuchet, all of which they had working knowledge of.
Do not forget about the longship as a weapon. Any settlement with access to the water could quickly reposition part of their force to place the attackers on the defensive.
The Vikings’ worst defeats were when they were caught or ambushed out in the open by a very mobile army. That shouldn’t happen in the fjords. 358