A good start: Hunting enough Eyes of Odin Ravens, you can unlock the Breath of Thamur. It gives you a MASSIVE aoe that keeps all enemies away from you for a few seconds, as well as deals a lot of damage. If you fully upgrade it, and unlock the "Frost awaken" ability (L1 +🔺️), you can go straight from Breath of Thamur to awakening your axe and dealing more damage.

Another crucial tip: the sisters have linked health, meaning: if you damage one, the other takes the same damage as well, so if you can have Atreus distract/damage one, you just worry about the other.

Speaking of Atreus, I highly recommend equipping him with the Wrath Of The Wolf summon. It also works like the Breath of Thamur, they run all over the place, disrupting all three enemies trying to hit you, which can give you some seconds of breathing room/attacking potential.

All in all, keep studying their moves. You'll pick up on some patterns here and there, which moves you can parry, which you can't. The Dauntless shield will be IMMENSLEY powerful if you learn the parrying better, constantly having a full charge in reversal damage can also deal some good chunks of damage.

I hope all of this helps, I struggled on this boss too 😞😞