How do you kill this so-and-so? Is there some secret vulnerability involved or does she just have a boat load of hit points? I'm using viper swords at level 9 but I barely scratch her before I die.

I killed a level 18ish when I was level 4 or 5. The easiest way I found to do it was to dodge and just cast igni. My normal attacks would only do 200-300. Igni on the other hand would tick for close to 2k when it lit her on fire. That's 2k per tick, and the burn would last for several ticks.

Easiest way is probably to use axii to stun it and hit 2-3 times and roll away. Dodge the crap she casts and when axii is back, use it again, rinse and repeat. Just killed a lvl 16 or 18 Bilge Hag as lvl 11 using this tactic on death march.

Are either of these with upgraded signs? For me Igni did nothing and Axii wasn't stunning at all.

No upgrades on my axii. I think it failed only once for me, could be that I was 2 levels higher than you.

She won't follow you very far, so I stayed outside of her movement range, stunned her and moved in with a few hits and rolled away.

The only points I had in igni at that time were 3/5 on the first tier, which shouldn't make much difference since all that does is decrease armor. I think igni burned her about 35% of the time. It isn't every time, but each time it burns it ends doing 30+ sword hits worth of damage, and by the time the burn stops you are close to being able to cast again.

I started out trying axii, but even with the first tier maxed out, it still only stunned her occasionally for some reason, and it left me very vulnerable for only a few hits at a time.

I ended up "cheating" on it; grabbed some loot and a key I needed. I unfortunately did not find the chest it opened but I'll come back later - at least it has a quest marker so I won't forget about it.

I'm at lvl 15 and took a lvl 25 Bilge hag using un-upgraded Axii. This broad would kill me with one swipe if she had to opportunity, so I kept quen up and Axii-d her which calmed her down enough for me to get in 3-4 good swipes using thundebolt and enhanced necro oil. Rolled back, loaded quen, then axii - rinse and repeat.

Took a while but she went down. Similar to taking on an Elemental that is above your pay grade. It just takes time and you have to be careful not to get bashed.