Well, Vikings are best described as wild. They rarely used complex formations such as a Phalanx or the Turtle as would have been used by Spartans and Romans. Vikings preferred a fighting style which focused more on a surprise attack. They just rushed into the battle as soon as possible and fought with a fever. They were the ones that Romans called Barbarians, which was generally used to describe how the Vikings fought. A Viking warrior would typically be in a state of 'bloodlust' which contributed to the Romans hatred for them. Vikings were almost the exact opposite of a Roman Platoon or Battalion. 

Now, this is not to say that they don't use tactics. Some Vikings used the normal tactic of waiting out their opponents. If they were entrapped in a keep, the Vikings would surround the place with fires and wait for them to come running out. They also took to hunting the villagers instead of rushing in. They would send a few Vikings in to terrorize the villagers and run them out of town and then they would have several archers posted around the village to pick them off. Either way, the villagers would die in the raid.