Let me give you tips, first buy these following treasure cards at the bazaar: 6 myth shields, 6 tower shields, 2 Leviatians,1 Helephant or 1 Jade Oni, and 2 Feint spells, if that doesn't work try using tempest, first you must have 6 power pips and you must keep on using feint to all enemies until you have enough to use tempest, after that use one storm trap on each of the enemies then use the balance blade, the elemental blade, then the storm blade, then use darkwind, then use tempest and the damage will be around 3760. If the boss uses a myth blade, disarm it with disarm, or steal it with steal charm.
Can you please allow up to four people to duel Vognir Runethief?
This is your school quest and must be done alone, but you can use a minion. It can be done. You just have to plan for it. And you may want to wait until you are at a level higher than 35, depending on your school.