The mundane stat is intriguing, and definitely requires a specialized build to make useful. However the results of making such a build are not underwhelming, and as such, a Mundane build should not be taken lightly.
Magic Magic Shield
First off is stats: Mundane cuts a weapons base damage in have and adds crazy scaling with your lowest stat. What this implies is that all stats should remain relatively equal to reap the full benefits. Mundane also hits a soft cap at 20, making each point after that half as efficient. Other stats will need to be higher for weapon requirements (Strength for Avelyn and Dexterity for Santier's Spear), and the rest of our stats should go into Vigor.
Another factor of mundane builds is castings. You have 4 natural attunement slots and respectable casting stats. However, a lot of your spells you're going to have to spice to meet the requirements. Not too many levels, so they should stay reasonably strong. I highly recommend sticking to hexes and pyromancies, as the sorceries are noticeably weaker.
Ring selection is largely up to the type of mundane build you want to run. Again, you have 4 Attunement slots naturally, and if you're going with hexes, that's enough for both Resonant Weapon and great Resonant Soul. If you're wanting Pyromancies, you may want to switch in a Southern ritual band to accommodate. Poise, despite popular belief, does actually work in this game. Having more with the Ring of Giants is helpful in making trades with lighter weapons, particularly with the Santier's Spear.
Armor choice is mostly because I like the way it looks, and is by no means super optimized. You can get some stat boosting gear and get a few extra stat points to spend, but in my opinion, you can't put a price on looking like a badass.
Santier's Spear is the bread and butter of this build, and is an absolute monstrosity. The moveset is versatile, and the multi hits of the 2hr1's and the 1hWRr1's hit for incredible amounts provided you get each hit.
Mundane Avelyn is the second crux of the build, and similarly does amazing damage. Off hand it to keep your opponent guessing and take advantage of openings such as empty swings and spell castings.
The Dagger and Shotel are backups: the Dagger for criticals and the Shotel for turtles. Both are fun to buff and use as a main weapon for a fight however; particularly dual daggers.