You can try go back to Father G's boss fight place see see?

I did started her quest, then I stumbled upon her fighting another NPC there...I tried to help but got killed, then she was killed as well. Sidequest failure ol

mine was using fast weapon. Ludwig's Holy Blade +9 with Bolt blood gem attached. plus blood gems that increase attack against kin.
Always try to hit the head when there's a chance, otherwise try to keep your distance and dodge. Once she closes in, dodge. Attack. Then keep your distance again.

Many ways to beat her, unfortunately she is also one of the most random boss in the game.

Some say you can cheese her and try to get in a position between the back of her tentacles. You can then just hack and slash away until she flys off and you just rinse and repeat until she is in phase 2. By then you need to start dodging and attacking when you get the chance.

There was also another strat where you can trap her near the corridoor's entrance. It's very risky as you WILL get hit a lot, but if your Vit is high enough and you can withstand her attacks, just deal everything you have with a bolt paper.

The Great Lake rune helps too btw, so equip it if you have it.