Originally posted by Cooperal:

Statistically all enemies are tougher as Dialtone said. In practice however most enemies are easier as one or more people can continuously land free hits so long as they are not the target of aggro. Especially during stagger/parry windows this can easily make up for the enemy health bonuses.

This is true, usually multiplayer is easier, as long as your playmates know which is the business end of a bow. Boss fights would benefit from some kind of mini boss helpers when there's many vikings, something to even the odds besides just bonuses.