You have to consider your opponent. Against Stronghold, any hero in the late game is useless; there's no chance of winning. Against Orcs, only Leto is viable, no other option. Against Inferno, Iranna works – her Furies either eliminate Cerberuses or Warhorses at the start of the battle. Then, the main forces attack the soft-skinned Demons. Against Dwarves, the chance of winning is minimal. Against Mages, perhaps Shadia is the best choice to minimize damage from constant ranged attacks, but even then, victory is unlikely. Against Humans and Necropolis, it's worth focusing on Chaos, upgrading Chain Lightning, equipping Rune Armor, and other enhancements. Erin, with her extra attack, could be a good hero for this. There might be a chance.
Against Orcs, I tried playing in the late game just for fun. I created near-ideal conditions for the Necromancer – the Orc doesn't have Slow, and the Necromancer has all the necessary artifacts. Instead of Assassins, I used Blue Druids from another castle, which increased the Necromancer's spell power by 30.
First, I tried leveling up the hero's Light magic. I wanted to focus on Divine Wrath. I equipped the Phoenix Cloak and leveled up Lucky Charm for a greater effect. I targeted creatures with the Water element. The result disappointed me.
Chaos was only slightly better.
However, races without resistance take 30-40k damage.
Regarding Leto and Orcs, there are no questions or other options; stunning is the most effective way to reduce Rage.
As for the Demon, Iranna is still debatable. Warhorses have an initiative of 16; they might go first and attack the Furies.
Chaos can still be boosted even in the late game; I think it's possible to reach 20k damage with the most ideal setup.