Need Help In a game.

Sp game. Remergence of Urgia I am pythuim and control all of south east cornor of urgia, vanhiem controls rest of the map. I am stopping vanhiem from invading me because I control a chocke point where I have many theurgs and soldiers and a doom horror. My god is an oracle who has 9 in everypath. I have researched level nine in all schools of magic, so I can cast every spell.
I am afraid if I invade, I will be overwhelmed because vanhiem has many many huge armies. So, what should I do to win?