I find, when faced with a massive invasion like yours, that it's best to consolidate your forces and take the enemy out one by one. Try and supplement your forces with mercenaries if you have the moolah. If you're heavily outnumbered, guerrilla tactics can help. Once I was faced with a crusade in my lands and I took out the enemy faction leader in a cavalry ambush, which tipped the odds in my favor and I repelled the invasion. Just some thoughts.