Like Bringing a Knife to a Gunfight.

AN IN DEPTH STUDY BY L** ****Y AND J***** ****E
After much debating the following has been concluded.

I. Ninjas kill Vikings:

The Ninjas advantage over the Viking is clearly his speed. The Vikings would be lucky to even lay a finger on a Ninja. If the battle lasts for any length of time the Vikings would become worn out and frustrated.

II. Vikings kill Pirates:

Vikings have a clear advantage with their catapults in which they heave giant balls of fire at the Pirate ships. Causing them to sink. Leaving the Pirates at a major disadvantage. And Vikings, having an unmatched pain threshold, wouldn't be slowed down in the slightest bit by the Pirate's bullets. And it's common knowledge that all Viking ships appear spontaneously out of fog, resulting in the pirates being caught off guard.

III. Pirates kill Samurai:

While the Samurai could easily take out a pirate in close combat, the Pirates have guns, in which they can easily kill the Samurai from afar.

IV. Samurai kill Ninjas:

Ninjas only have one weakness, the Samurai. The Samurai, all being well trained warriors, are the best by far in close quarter combat. Since they are always prepared for battle, the Ninjas lose their stealth advantage.

References:

Mulan (1998)

The Last Samurai (2003)

Pirates of the Carribean (2003)