Prepare to dodge a lot, but also play aggressively at the right hitframes. I used light armour for this boss (as I did for all bosses, basically) since you won't be able to tank her hits anyway. I used the Gefn Blessing relic, with talismans that increase stamina regeneration and decrease stamina costs. After dying a couple of times I equipped the Fury talisman instead of a health regeneration talisman, which increases damage output after your companion dies (which with AI definitely will happen because your AI is a moron).
I fought with a Lathyrian War Axe, fully upgraded, paired up with Malik's shield, and Celestial Aegis armour.
Path up to the bossfight
Stock up on White Sapotes for the small number of enemies faced when making your way towards the boss fight. You should save all your swigs for the boss fight, in case you make mistakes (especially during the Second phase).
The Boss fight - phase 1 - Aggressively
In this phase, your AI makes a good distraction and tank while chipping away at her health. I recommend playing aggressively in this phase, because two hits of my axe stunned her already, interrupting her attacks. Don't get greedy, however. She's quick with her attacks that do pack quite some punch. Wait for her to use her overhead smash move, which she does quite often, especially when there's a small distance between you and Sissna. Your AI will probably survive this phase easily, especially with you stunning her in her back while she's trying to wreck your AI. As soon as phase two starts, the real difficulty begins. I usually ended up not having to use a single swig in phase one, which made phase two significantly easier.
The Boss fight - phase 2 - Patiently
Pull yourself together - the real fun begins. In this second phase, Sissna is even more aggresive and especially more hard-hitting than her weak phase. Your AI will probably die very quickly because of his stupid aggressiveness at moments where you should be dodging (which is why switched to the Fury talisman).
Don't even think of actively blocking her attacks (at least that didn't work with my shield - too low stability) since it drains your stamina rapidly and probably breaks your guard before she's finished her devastating attack combo's.
Staying close to her proved to work well, as it makes for a good hitframe after her missing attacks. However, those attacks show little signs and therefore require being highly alert. Another good hitframe is after her charging attacks (after her running around like a moron or her other charging attack, which you'll easily start to recognize). Sometimes she will charge herself, which does a little bit AoE damage, followed up by a very powerful but slow 'walking-like' hitmachine. If you brought some powerful spears, this is a moment to throw 1 or 2. The hitting spears will interrupt her attack though, so be wary of her coming towards you quickly after the spears hit.
Be patient, and throw in some good punches at the provided hitframes, and you probably will need most of your swigs due to being greedy when attacking or being too slow while dodging.
Good luck with the hardest boss fight in this game!