Just finished the game in 2016 again in hard mode. Here I have a non-cheesy honorable way (without blowing up the undead's base) of winning Twilight of the Gods mission.
The key to win I think is to have massive dryads with a few hippogryph riders. That's the only way you are able to survive the hardest waves of undead army, which are
- Hardest: undead hero with infernals, hell hounds, doom guards etc.
- Abominations and necromancers.
Bears (druids of claw) are vulnerable to both mana burn from hell hounds and cripple spell from necromancers. Huntresses are generally too fragile.
Dryads are perfect for the listed 2 waves because of its spell immunity. As of the damage output, the slow poison can greatly slow the entire process of combat, during which Furion's Tranquility spell can increase their survivability significantly.
The added hippogryph riders can boost the damage output quite a bit, especially with the true shot aura and focused fire. And the undead army has much much more fire power to the ground army than that to the air. If you have extra APM, the hippogryph riders can be easily pulled out of combat to the fountain of life.
The ultimate wave of undead is launched roughly 15 minutes after you move to a new base. Hence if you can defeat one or two ultimate waves, the victory is yours. The easiest way to accomplish this, is at Thrall's base thanks to the goblin mines. You should always purchase the mines, and probably better save them for the ultimate waves - the hell hounds, doom guards and infernals. Infernals are the most difficult unit here because it's spell immunity and large hit points pool. Save the mines for them.