The Viking is a slow but also very powerful destroyer that excels in naval combat.
Very high durability
Very high firepower
Torpedoes allow it to engage Barracuda submarines
Defenseless against avia
Not very effective against infantry
Vikings are relatively easy to get as they are available at HQ2, which makes rushing them a viable strategy. This is especially the case if one considers that their firepower, health and armor significantly exceeds that of Zeus. This makes it a huge threat to any ships, vehicles and buildings. It needs to be noted though, that Grenadiers and Heavy Assaults pose a threat to them, as infantry units trade pretty well against them and that they are completely vulnerable to avia units like Hawks and Dragonflies .
This vulnerability to avia units also means that they should either be used in the beginning of a battle as part of a rush before the enemy can get out their avia or in the late game when they can be supported by Poseidons which have a good anti-air weaponry. Deltas are also an option when the avia unit used is Dragonflies or Wasp.
Other than that, it is also possible to use them purely for defense and keep them close to Torpedo Platforms which have the second strongest anti-air weaponry in the entire game or keep them close to the coast where Typhoons can be placed as anti-air support.
Experienced players can also use well-timed Vertexes to snipe Hawks or Dragonflies attacking a Viking. It needs to be noted here that, while Viking is defenseless against Albatross too, it needn't be feared that much due to its high health and armor values.
Part of their firepower comes in form of torpedoes, which have an insignificant travel time to their destination and lower range as the Viking's main gun. This has various effects:
Sometimes torpedo shots will be wasted on targets that would have been killed anyways
Viking is the only unit in the game that can directly target and attack submarines, trading very well against them
Torpedoes can't hit land targets, therefore reducing its firepower against land targets