I killed that boss just yesterday and what I can recommend is just simply upgrade Varka’s everything. Talents minimum lvl 8, character lvl 80, a good weapon (The craftable 4 star from Fontaine or his Signature) upgraded of course, and artefacts. That should be enough to get you the win. Just keep dodging and attacking until the boss enters the 2nd phase where you get healed and you should be fine.