I have completed the highest difficulty (Witch Maze 8 in Version 1.1.25)
Any Strategy worked for me up to Witch Maze 4, these are my strategies for Witch Maze 4-8

I have found that I can make most rounds a winning round with these strategies, even though you are shown random units/gems/shops etc you can generally follow these strategies (as so many are shown, it becomes at lot less random as you get to further floors) but every game will look different.

Unlike other auto-battlers this game is about synergies (which I think is a good thing), while 3* units may be fun and good at lower difficulties, at the hardest difficulties with less gold it is very unlikely to get a 3* if you were trying to, and even if you did, your team would be weak compared to the synergies you would have gotten (synergy bonus + green synergy health)

Amelie:
Early Synergies: Knight, Brown, Plum, Blue
Late Synergies: Green, Orange
Extra Synergies: White, Star, Wheel of Fortune, Magician
Final Units: Forest Mage Sphene + Another Healer + Sun Knight Sol (or any gold, recommend her for scaling damage) + 3 Gold Units (all level 2 if possible)
Final Units Alt: Three Healers + Dancer Reno + Any Two Gold Units (all level 2 if possible)
Preferred Items: Aegis Earrings (No damage penalty on Shielding) or War God Earrings (+5 Attack when targeted) , Haste Necklace (Front unit has double action), Any Neck
Spell Gems: 5 Gold Shields. Thorn > Agito > Arcane > Iron. (Required for final boss)
Notes: All about sustaining the hits

Zeola:
Early Synergies: Orange, Beast
Late Synergies: Green (Minimum 5, preferred 7), Emperor, White, Stone, Yellow
Starting Units: Thunder Spirit
Final Units: Thunder Monster, Magitech Golem (Green Synergy), 4 Units that heal (all level 2 if possible)
Preferred Items: Need green on at least 1 piece for green 7, Mirror Earrings (Skipping makes next gem cast twice), Haste Necklace (Front unit has double action)
Spell Gems: 5 Gold Shields. Arcane >Thorn > Agito > Iron. (Required for final boss) + Magic Armor or Heals
Notes: Zeola is a lot harder than Amelie as she does not have access to as much damage mitigation/healers. Shock damage goes through shields

Strategy:
Be ultra greedy on floors 1-3
Get a Healer early for sustain
Fill Spell Gems on Floor 2, 6 (Silver), 10 (Gold) (Note this will be one floor higher on lower Witch Maze Levels)
Pick any spell gems on floor 2 + 6, save your crystals for floor 10 and make sure to get 5 shields
Do not reroll the Trader at all until you have unlocked all character slots! (gives you more room for keeping characters and gives you more power early)
Never take events with penalties (Except on floors 1-3)
Sell level 2 units except for your final units. (or keep for a boss fight then sell if you are lacking power)
Do not try for level 3 units (too costly, could have gotten many more synergies instead)
Make as many synergies as possible, prioritise early game synergies above until you feel powerful then start prioritising late game synergies.
Pretty much fully explore the floors, don't be afraid to take some damage walking without mana (unless your on a boss floor)
Shield every turn past floor 10