[Alneyan]

The Sceptic reduces Dominion. All Dominions, yours included. I do not know how quick it works though, but a couple of them should be enough to break an enemy Dominion quicker than you can say "Inquisitor".

Caelum is the one nation said to be overpowered, though a few others are Banned from time to time, and water nations depend on the map (you do not want to have both Atlantis and R'lyeh competing for three sea provinces). But in the case of Caelum, the problem seems to be more the False Horror spell than an actual powerful nation.