Magistus is a deck of Level 4 Spellcasters, while Vaalmonica can Pendulum Summon Level 4 monsters and has a Spellcaster.
The deck takes advantage of The Three Champions of Swordsoul, which can search any Level 4 Spellcaster with ATK = DEF, including both Selettrice Vaalmonica and Spenta Magistus.
The Vaalmonica bridge
Once you have Selettrice, Angello, and Dimono on the field and enough counters:
Turn Dimono into Requiem and Special Summon Engraver.
Attach the Requiem from the GY to Engraver.
Make Magistus Chorozo using Selettrice and the Requiem in the Spell & Trap Zone.
Synchro Summon The Three Champions of Swordsoul using Chorozo and Angello, and add Spenta.
From there, you can add Crowley and, using him and The Three Champions, make Fusion Zoroa. Use Spenta in the GY to equip Aiwass to Engraver, make Necroquip, and finish the Fiendsmith line.
You will be missing a body for Zebufera, so either you have to Pendulum Summon an extra monster (a Charmy or a Vaalmonica body), or find another way to provide the additional body.
The Magistus bridge
You can add Zoroa with Spenta in hand.
Normal Summon Zoroa.
Use its effect to Special Summon Spenta, then Synchro Summon The Three Champions of Swordsoul using both monsters to add Selettrice.
You can then use Spenta in the GY to equip Artemis to The Three Champions and add Crowley to finish the Magistus line.
For this bridge to work, you need to already have access to the Vaalmonica Pendulum scales. Vaalmonica doesn't have a one-card starter, so bridging into the deck is kind of complicated.
The Vaalmonica line is a little different because you don't have your Normal Summon available, so you have to Pendulum Summon Selettrice and use Invitare to Special Summon from the Deck (so you can't do the Fiendsmith line because of the lock).
Of course, if you have access to both engines, it's much better. You can use Spenta to add Zoroa, do the classic Vaalmonica line, and when you reach the point where you Pendulum Summon, you can also Pendulum Summon Zoroa so that you can continue both lines.
Is that deck meta-relevant? Absolutely not.
But is it a valid alternative to Vaalmonica Fiendsmith? Maybe.
I just added some non-engine cards that I like to finish the deck. I don't think they're optimal, so feel free to experiment with them as you like.