So, you need to try to find ways to win based on the actual situation in the game.
2Psi
Against the Heavens, you can come up with something:)
As an option:
1) Imps in the spellcasters.
2) Deadly Cold (with the perk). With the new Druids, all Chaos magic will work well:)
3) Fire spells (with perks for fire in Chaos magic).
4) Dark magic.
5) Blinding of unique units.
6) Lightning (with the perk).
7) The new archers are quite good against the Heavens with their enhanced arrow.
8) Artifacts for initiative.
9) Light magic (buffs and Vengeance - which can severely damage them, considering they are a damage-dealing stack).
10) Morale and luck.
The Heavens may have a better chance of winning in a long game, but that doesn't mean anything:)