Alright yeah that fight is nasty. Probably a good idea to wait until level 12 if you aren't already. Beyond that:
The enemy only has three serious damage dealers: Viconia and the two Justiciar Knights. The Sharran Novices and Sentries are irritating from their numbers, and the Darkness that the caster guys spread is irritating, but those three are the only ones that'll shred your party.
Battlemaster Lae'zel is capable of killing Viconia in one round by knocking her down with a trip attack and then dumping her superiority dice and Action Surge with Great Weapon Master:All In active. Using the Soulbreaker ability from the Silver Sword of the Astral Plane can get you an extra round to bring down Vicky. Her saves are remarkably unimpressive for such a high-level character.
The Justiciar Knights are also easily disabled by a Battlemaster, especially if you have the Gauntlets of Weapon Mastery. Menacing Attack is more consistent than Disarming Attack, since they have better strength saves than Wisdom.
Daylight is a third-level cleric spell that dispels magical darkness and emits bright light. This is the one time in the game that it's useful, as not only is clearing the Darkness spells important, but a lot of enemy abilities in this fight don't work in the light. Luckily, clerics automatically learn the entire spell list, so make sure Shadowheart prepares it going in. Remember to remove it from your preparation list afterwards.
Speaking of Shadowheart, pre-casting Summon Planar Ally is a good idea. The Deva works well both to absorb hits from the enemies and to chase down the casters to break their concentration on Darkness.
Shadowheart is not a healer in this fight. Not only do many of the enemies have Bone Chill, which can prevent healing, but also she has too many other jobs to do. If she doesn't need to clear Darkness, she should be annihilating a group of enemies with Flame Strike, or nailing a caster with an upcasted Guiding Bolt.
You need a lot of AoE in this fight, so I recommend Gale. Ice Storm is ideal for taking out the hordes of schmucks, as they're weak enough to often fail their saving throws, preventing them from approaching.
The Sentries are only 4th level, which means no extra attack, which makes their damage output pitiful. Even Gale can easily take hits from them.
Kill order: Vicky -> Knights -> Casters -> Schmucks
I won with a party of: Tav(Half-orc Battlemaster)/Lae'zel(Battlemaster)/Gale(Invoker)/Shadowheart(Trickery Domain)
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