Raids were different than battles. You can't mix the two together. They went on raids planning for little resistance. The best advantage Vikings had for raids was surprise. They were meant to show up out of nowhere, take what they want and get out before anyone showed up. A group of 20 or so men on horseback could probably chase away a small fleet of Vikings from the shoreline. Not because they were scared, but because they went to pillage, not fight battles. If loot was their goal and they had to fight for it then they would often just travel somewhere else where they could get what they wanted without having to lose any Viking lives. Viking battle tactics on the open field were another story however. If you want more details on that, then maybe someone who knows more about actual Viking warfare can help you.

As for raids, a similar question was answered here. http://www.reddit.com/r/AskHistorians/comments/1r01iw/was_there_such_a_thing_as_a_typical_viking_raid/

Because the where clever and opportunistic. When defenders proved to be to well organised to to fast to react the vikings stopped attacking and started trading. Only if the enemies where 'easy' to defeat they would fight (at least on raids). Note that there are not many records where they storm castles, they did not have many siege weapons and the did not have time to hang around.

There is a reason the most attack structures where monastery , they where rich, the monks did not fight and the did not have defences. The vikings would land go into a monastery steal the valuable stuff (religious relics are specially nice), then maybe go to another one and then go home. This is not easy to fight because countries like France or England have a long coast and lots of rivers, the vikings could pop up anywhere even far inland.

To fight this effectively you either need a navy, or defences on the rivers and ports, lookouts and a mobile army that is on call. This is really hard to do and the english and french kingdoms simple where not able to do it, and the vikings made us of that fact as best as the could.

The vikings also attacked Constantinople but the figured out quickly that the where to well defended and organised.

Later in Vikings times the did much more and where also involved in large invasion and settling (for example Normandy and Danelaw).

Note: Watch the TV show vikings, it shows pretty well how the would just march into a town and where gone the same day.