Ok so, Siofra is actually just difficult - the first real boss so to speak (sorry, abomination). SO don't feel bad if it takes a bit of effort.
The catch with her is that she has 3/3.5 phases, and you want to play them carefully.
In general, take down the summons ASAP, if she phase swaps while one of the wisps is up you're probably gonna have a bad time.
In her final phase, she gets the infinite scaling version of Smoulder you have access to, so you're on a time limit. The tankier your party is the longer you have to burn her down, someone like Sarah can help with survivability, but in general you have to nuke her as fast as possible. Stuns help.
Since Siofra and her summons do pure magic damage, it may be worth trying Unarmored for the extra mana regen to afford an extra channel (maybe), or looking into any resistance traits if you don't have them? One of the new shield abilities, when mastered, also grants an OK AOE heal that can help you top up going into phase 3. Make sure you have any source of healing/regen possible, like Lifestream from water magic and the regen trait.
Party-wise, I did it with Sarah, I've heard Aspasia is actually really good against her, and Ashley might be good for the lifesteal/poison/damage debuffs as well.