Well Aslan and Gandalf the white are allegorical to omnipotence, but they don't show it in series. We are ignoring the differing makeups of the differing universes. There are four portals in key locations in each of the planets (Nirm, Earth, and Arda) those are the only means of going through each universe and they cannot be sealed until one of the sides is defeated. Each side is given 50 years of prep time (meaning the UE has time to study these newly acquired items and potentially improve them and mobilize this gives the evil factions time to homogenize) The UE has been given all the relics, weaponry, records, books, magic, and so on left by the 'good' factions in all the series.

ĮDIT: -the evil factions have lost their Godlike, demigod, unkillable universe/world ender beings, creatures, etc. The evil factions from the the other universes also are united as well and the 'good' factions from those universes have mysteriously disappeared, but have left behind their books, relics, weaponry, etc. The battleground is primarily middle earth, but there is a possibility of invasion to the other realms via portals. and billions manpower) to battle the collective evil factions from those universes.

In this 'VS' the entire planet of modern earth has somehow united into a collective force (means shared tech., science, etc.