I've only once been able to beat it with Vikings, for it is indeed very difficult.

Problem is that you don't have area of effect damage spells, so you need to smartly combine things to beat large groups. Like throwing a snowstorm slightly in front of the group, so that the group stacks up on a single spot, and then Raging a couple of Frosthammers at the group. The Rage impact does a lot of damage to the entire group, as do their hammers.