the fight is part of the main story so just do them and you'll get to it

It's basically just a matter of killing as many as possible to reduce the giant burst when they summon him and then keeping him away from the group while you deal with adds in whatever order threatens your group the most.

the king seems to just be a big tank that does instant aoe's around himself
bard charms party members
white mage heals
black mage does a really huge aoe around himself
the archer marks targets then shoots them for a fairly hard hit
the warrior basically holmgangs people

not entirely sure what the paladin and thief do though they seem less noticable than the rest other than the paladin melees fairly hard