Had to do it twice because I found out Eryka is the only person who knows how to tackle. For those who want to know how to do it, here's what I did
Gear doesn't really matter, I used the witch set the first time, and leather armor from the shop the second time. Your damage is inconsequential for the most part. I was a Black Mage, Grease is a very useful ability. Bring Cait and Brint
Cait leads with Sun of Jazirra or Heal depending on if Sigrune gets initiative. Hopefully the blind works and Sigrune misses whatever over powered attack she goes for. Brint needs to go for Rend first turn and it needs to hit and get the bleed. If you miss or don't get the blead, reset. Use your turn to cast grease and get Sigrun to waste her turn standing up, and have Cait on heal bot duty. Brint needs to Cleave next and it needs to hit. Next turn, summon the fire elemental. Best case scenario, it gets a hit in. Second best case scenario, it eats a hit for you and Brint. Brint now needs to go for Execute. It MUST hit or you have already lost, since it's a 200 damage nuke and greatly evens the playing field. After this turn, someone is dead. Best case scenario, it's the fire elemental. Worst case scenario, it's you or Brint. If it's Cait that's alright but get ready to clench those butt cheeks. At this point, you need to give Sigrune everything you've got. Use a leannestone if you have one, or use lightning bolt. Have Cait heal if she's alive, if not, just have Brint use his ability attack and hope for the best. If you didn't die, go for withering bolt or lightning bolt. If she's still not dead, hopefully Brint died for you. Now just use withering bolt and pray to whatever God you worship that it works.
I really hope we get the ability to bypass sigrun soon because it's hooonestly garbage that there are so many missable skills locked to mirror stance behind this level 5 behemoth of a boss.