Many people have had problems killing this spider.
All the forum posts are a year old. I am playing NG+ and keep on dying.
Most people appear to have not noted the level of the quest "Missing brother"
and failed due to their levels being much lower than recommended.
I am two levels higher than recommended yet am unable to survive.
I am about to start the final battle and all available side quests are way too low
to bother with so leveling up is not an option.
I consume golden oriole, thunderbolt, use insectoid oil but my health drops
too quickly to survive. I could strike and run if it was not for the other spiders
constantly attacking.
I need to decide whether to try again or give this quest a miss.

Rule number 1: do NOT get into melee combat with it. Hopefully you have a point in Magic Traps in Yrden. If so, use them, they keep Harissi at bay, and you can get a hit-and-run attack while it's momentarily stunned. You may also use Firestream if you have that in Igni. Melee only when it's safe to.

I watched a video where they also used Yrden and they took refuse behind a rock. Harrisi did not come near the rock
so that's something to bare in mind.
They also used firestream which I do not have. The fight took 15 minutes.
I guess I better go back and try again.

UPDATE: I finally succeeded. Harrisi was relatively easy. It was the half dozen babies ganging up on me
that proved difficult with my health bar dropping alarmingly in a second.