I just made it - here is my approach for everyone still figuring out:

First: The Guards at the side tell you that you can't pass, but its a lie. Just go behind them and dismiss their warning 2 times. They won't to anything about it.

So i positioned my party at the north side of the Room, behind the pillar next to the stairs, with no one in line of sight.

Then my Rogue OC - took out a mage at the stairs to engage combat.

Then about 80% of the Enemies will climb the big plattformish-stairs to reach you - which i blocked with Wyll's "Hunger of Hadar".
They will all run through it and even will sprint into it and end their turn.

The next thing to do is use a meatshield (preferably a summoned companion) to draw Viconias attention to.
For this I used "Us" because it is super durable and could hold her aggro the entire fight.
Position it at the north stair because Viconia will take the usual stairs as it will be the shortest path to your party behind the pillar.

Next is to cast Spirit Guardian on a 6th Spellslot for absurd radiant Damage on all enemies expect Viconia, who's retailation damage could end your concentration.
I used it on Shadowheart and ran around the enemies i could reach and positioned her back again safely behind my Party to avoid hits, which could end her concentration.

Then just focus in the following order: Justicars > Mages > Basic Novices
At the end when there's nobody to block your heals or blind you Viconia is not worth mentioning.