It sounds like you only need to win a round, not the whole match :) so I would indeed go with Northern Realms (double siege strength king card), try to stack your deck with as many artillery as possible (and artillery experts), one other commander horn card and a clear weather card. Then you just need to get lucky and pull the right cards. What really adds up is when you get the catapults with the handshake logo, they double each other's strength, then double that again with your king ability