posted this elsewhere already, a couple years ago .. it's probably much too late to help you two, but maybe it can help others .. anyway, copy-pasting what I've already posted;
" Google brought me here, in search of an answer to that very same question .. I just managed to beat it, by simply marching past the enemies, with Sigurd leading the charge, he's wearing plate armor, so he can take a little more punishment than the rest
enemies seem to spawn endlessly, so there's very little point in fighting them - when you kill a group, replacements spawn from the center of the map, and heads out to replace the ones that were killed
only enemies that are STATIONARY seem to aggro, so those that are moving around the map, you can simply ignore and move past, they shouldn't follow you .. the enemies that do aggro, seem to favour attacking the weakest party member - with regards to either HP or armor
the mission in an off itself is over in ~10 minutes, maybe less if you just march straight into the center .. you'll need all 4 heroes there, for the mission to complete; even if the area is flooded with enemies it doesn't matter, you'll still win "