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When I play as Nature, for example, I take 6 heroes with the following starting skills:
* Chaos Magic
* Chaos Magic
* Scout
* Tactics
* Magic of Life
* Magic of Order or Death (usually buy in the first captured town)
First, I level up the initial skill group for all heroes.
Then, I level up Chaos Magic for everyone.
Then, I level up Order for everyone (to slow down and hypnotize).
Then, I level up Death (for Sacrifice and Death Grip).
As soon as I level up Chaos Magic for everyone and Order for at least one hero to the point of mass slowing, I start massively eliminating level 4 creatures, plus dragons and water elementals, which I already have quite a few of.
I level up to level 4 as much as possible.
In the final battle, I eliminate heroes with a storm of Chaos Magic spells and also eliminate dangerous enemy units with hypnosis and sacrifice.