Notes & Combos
Magistus are fun! I didn't actually use the extra deck out of archetype cards basically ever, they're mostly just a toolbox just in case. The fusion and lower rarity magistus XYZ (Aiwass and Ninaruru) you could probably go to 1 copy of each with no downside, but there's also not much more you need in the deck, though a third Artemis might be nice.
If you're building this outside of the Theme event, swap 3 of the non-archetype xyz of your choice out for a Zoodiac Drident engine, swap Omega for Savage Dragon, and swap 1 of the 2 ofs or the remaining generic xyz for Zeus.
For the combo lines and the likes, mostly just follow the combo lines from the Magistus guide on here. You'll get the hang of it pretty quick - goal is just getting Calamity to be big and hard to deal with and winning through that. You don't need a ton of cards to do your combos, so you have a lot of room for generic going second cards to let you have a chance. Divincarnate never really came up tbh, I've cut her numbers multiple times and will likely remove her fully eventually.