It's the Avengers: Endgame final battle. But both sides have a few new recruits.
Joining the heroes:
Superman (DCEU)
Batman (DCEU)
Wonder Woman (DCEU)
The Flash (DCEU)
Aquaman (DCEU)
Cyborg (DCEU)
Shazam (DCEU)
The Suicide Squad (DCEU)
The Amazons (DCEU)
Joining Thanos' side:
Zod and his henchmen (DCEU)
Doomsday (DCEU) - and yes, I know this doesn't make sense, given that he was made from Zod's corpse...
The Enchantress and Incubus (DCEU)
Eyes of the Adversary (DCEU)
Ares (DCEU)
The Parademons (DCEU)
Black Manta (DCEU)
Dr. Sivana and the Seven Deadly Sins (DCEU)
No Infinity Gauntlet involved.
Every character involved in the original Endgame battle is there from the start (Captain Marvel included). In the case of generic characters (Amazons, Parademons etc), just assume every one ever seen on screen is present.
How does this fight go now?