So I struggled quite a bit with the Primary Challenge, so thought I'd show the builds I eventually managed it with.

The Logic:
The best way to survive an encounter with Satell, is to either stop him from hitting you (by petrifying him), blocking his damage (through energy shield and primary protection), or tanking his damage through large buckets of health, kill him before you can hit you (Empowered Maximized Heightened Big Bang) or just plain not getting hit in the first place (~1500 dodge).

I will be assuming all other storylines unlocked.

Death/Space:
I actually tested this build after completing it with Air/Poison, but it felt much easier. Pretty hard to screw up. Focus on studying Space till big bang, then focus on studying death till Bountiful ritual, then Space until you get the Cosmic Wand and eventually reach level 80. Boost Space to max furniture.

Fill your pouch with mostly potions of revival (from the white dragon boss), and equip the robes of death, synchronizer and mithril/crusader shield for some extra defence. Use Wand of Doom + Soul Collapse for general mob item farming, use Cosmic Wand + Big Bang + Nova for Bosses and Plains of Peace Item Farming and Satell. Spellvat Death Jewel onto Big Bang, Boost using Holy and Mind to max, for Spell Damage and Accuracy, activate all spell catalysts.

Use this Syncro

Then you should have ~9 mil big bang damage and can kill Satell pretty easily.

This Air Poison build aims to raise your dodge as high as possible, through items and the blur spell.

Choose, Air, Poison, and the Sage class. Air has Blur, Poison has Sewage enchantment, so you have some essence income once storage expanders come online, and together they have some good dodge items. Sage boosts Blur.

Use this Syncro to boost Sewage once you have paradox so furniture takes less time.

You want something close to this lineup before taking on Satell
Shadow Blade +15, Ninja Master's Gear +8, Ethereal Footwear, Cloak of Darkness +15

Potion of Revival (from the dragon boss)
Clever Pumpkin
Revitalizer (from Plains of Peace)
Dispelling Salve (also from PoP, I'm not quite sure Satell gives debuffs, had it just in case, can possibly replace with Revival or Revitalizer)
Greater Potion of Toughness/Muscle ~+5 (from Chaotic Path)
Potion of Evasion (crafted from starting shard stockpile)
I also have a Potion of Alacrity, but in hindsight should have used those water shards to craft more Potions of Evasion. Hold off on crafting the potion of evasion until your Poison is relatively high level so you have a better chance of crafting high level potions. Replace with another Potion of Revival probably.

The Ninja Master's Gear will probably take the longest. It's just slow. Recommend putting it on autocrafting when you can. If you can get the Gear higher than +8 you can probably slack off in other areas, like studying, so it's helpful if that's easier for you.

Generally I use this synchro setup for item grinding, helps a lot.

The Synchro Setup I use for crafting.

A checklist:

I studied Air and Poison to 80 for the spell bonuses (mainly to blur). But this takes a long time. Possible level 70 is enough if you have higher level Ninja Master's Gear.

Boosts

Spellvats
In retrospective hindsight, I might have switched out some attack for death and possibly put a second boost on Blur. My wizards were kind of tight. Can't remember if I tested this.



Satel Bashing Synchro

I thought I took a screenshot of my final stats but can't find it. From memory, you want about 1500 dodge for Satell to not be able to hit you.

And then you should be able to kill Satell. Yay!

There are probably several other ways to beat Satell. I'd say the chief bottleneck is actually essence production and elemental shard/prisms/jewel making. Or maybe I just play in a silly way.

Builds I have tested:
Mind/Space: I was doing this for the accuracy boosters. Don't recommend.
Holy/Space: Theoretically, this might work with various boosters on big bang, but essence gain was incredibly slow so I gave up.
Earth/Poison: Ol reliable boosted Petrification + Poison. But without access to most prisms and jewels for duration boosters or cooldown reducers, I couldn't get petrification to be permanently on and Satell kept turning me into a paste.

Theoretical builds that might work, maybe?:
Death/Holy: Death's bountiful ritual solves the essence gain problem, and then with Primary Protection, Potions of Revival and the Rainbow Mail you can possibly just tank Satell's attacks and kill him slowly?
Air/Life might work, with the Shapeshift Mouse and Druid's Robe. But essence gain would be very slow. Haven't tested at all.