A build suggestion
Druid is one of the characters that can best utilise spirits, Elementalist is very useful for skill combinations. With them you can build quite a dangerous build through Fusion. Though I ain't going to be going in depth with the stat and breeding
Start with an angel at least to try and get "Martyr", then perhaps you can fuse an Angel to get a Harpy to easily mass breed unit with Martyr
You should also have "Archived" trait and a Manaspark on your starting Angel
Using the gallery, spawn Narah and get her "Reconstructing" trait.
With a "Manaspark" and a "Reconstructing" on a unit, it can spam "Martyr" every battle, fully healing your unit every time
However that ain't nearly enough.
Spawn a ice spirit and evolve it to "Frost Weaver" to get "Immunity" on a unit,
Fuse the unit with "Immunity" with one of the newly hatched "Martyr" unit to get "Sacrifice".
Fuse one of the "Sacrifice" unit with a spirit with "Reconstructing", "Manaspark", and perhaps "Intelligent", "Arcane" and Manabound, ensure that your unit has at least 25 max mana
Fuse that Sacrifice unit with another "Martyr" unit to get a unit with both "Sacrifice", "Martyr" and all the aforementioned traits.
Now you have an immortal units that can be killed for full mana or HP every turn.
The next part depends on how involved you want to be.
2 unit can get a Meteor Storm
4 can get World End
8 can get Apocalypse
You want to get an Apocalypse, but you can probably breed it out. It depends on how much you want to do with Fusion.
Spawn a Fire Spirit and evolve it into Elemental Seed with "Flame Strike". Fuse a unit with that spirit to get "Flame Strike" , Fuse that unit with another Elemental Seed with Flame Strike to get Meteor.
Each unit can get a Meteor. You can breed or fuse but the final goal is Apocalypse.
Play Druid normally and get 3 or 4 Seedlings monster and 1 Seedling Dragon with 6-7 trait
Use the Seedling Monster and Dragon (at least level 16) to rush the Avy quest to get Flake
From Flake get the "Robust" trait.
Adapt "Robust" onto the Sacrifice/Martyr unit with the Reconstructing/Manaspark/Mana boosting traits
Get at least three "Apocalypse" unit with "Robust" and all the Magic boosting traits possible.
Optimally, you will have three 100 magic caster (first turn only) unit with Apocalypse.
The three can do 900+ true damage ignoring armor every battle, which is enough for nearly everything in the game.
Their only weakness is their mana/HP, which can be mitigated with the Sacrifice/Martyr unit.
Even in the final battle with the Overseer, you end it with two hit, and the final "God" battle can be solved with a single "Sacrifice". After which every unit except the two "Gods" die to a single Apocalypse.
Due to "Robust" they can't beat down the three unit fast enough and they will die next turn.
Edit: the most important thing is how to manage the HP/Mana trait of.
A good combo is Sacrifice in one battle/Martyr in the next/ Sacrifice/Martyr/etc
In fact there are much simpler way to beat the game, this build is just a response to the game new extensive Mythic portal.