So apparently the piercing wand lowers resists past 0, into the negatives.
I decided to see just how powerful this was. I did get a little lucky along the way as well, but...
Unfortunately, I didn't take enough screen caps for a full walkthrough, but here are the highlights.
TLDR: Scroll through for the screens that I did get.
Decide to try Dragon Isles to test myself, as it is the toughest Vicious dungeon for me.
I chose Elf Sorcerer for the extra 5 MP, and because the Sorc is my favorite class.
Prepped: A whole bunch of stuff, but the important things are...
Fireball (Sorcerer practically needs this prep)
Things start out a little rough as a magic user with a 7 cost Fireball and all enemies having 15% magic resist. Luckily the Sorc excels at the early game. Oh, also? I find BLUDTOPOWA in my first 25 tiles or so.
I fireball things left and right before and after finding Mystera who is all the way around the back of that starting wood grove. Take the first +MP boon, then FB my way to Mystical Balance. Things are starting to get a little easier now that my FB drops 2 MP/cast, and they get even easier after I take the first Weakening boon.
I find Firstborn and I am level 5. I decide to go for it.
I regen fight the Firstborn down to -79% magic resist before striking the final blow. At level 5. With at least 30% of the map still covered. So far so good, and all this without the Crystal Ball I usually love so much.
This is when I catch the second lucky break of the game and find the Crystal ball in a store. The Matron is in for it. I descend into her lair and then realize that weakening does not penetrate underground. Oh well.
I regen fight my way to the Matron, killing all the guards and still not having hit level 10. The Matron starts with 65% Magic resist due to my Mystera prep, but ends at -52% due to the Piercing wand.
At the end of it all, I was still level 9 (After killing the Matron), got Feeling Parched and Specialist (WITH MAGIC!)