I think I have found tactics.
Have at least 3 long range level 15 in party(that hasnt finished shadow quest), recomend everyone have health potions.
There are 2 bookcases at edge close to shadows is safe spot for party.
have one member attack green or violet shadow depending on which side your on, then that member runs back to safe spot. At that point all long range attacks the only shadow that will still be attacking. Melee fighters can still do damage they just need to do hit and run tactics, when other shadows start coming towards party because of melee fighters they need run back to safe spot.
The most important part is to not leave once you get mirror or else you can't help your party.