Before unpausing, set your initial stack to move to Harrenhall, if you want an event. There is a larger Lannister stack there, but it will disband as soon as the game starts.
With any luck, the ironborn will declare war on you. This is what you want because now you are fighting a defensive war. The latest patch gives your vassals an opinion boost and that means more troops for you, over twice as many as you had at the beginning. After the event at Harrenhall fires, move your stack to your territory. Disband the stack and raise your levies again. Order all of your troops to group up at Winterfell.
You are completely ignoring what is going on in the south for now. While your troops are moving to Winterfell, the ironborn will land somewhere in the north. They have less than half your troops, so start chasing them down. Be careful of attrition and make sure that you are using a couple of stacks to corner the ironborn. Wipe their entire army out.
Meanwhile, the Lannisters are busy sieging your lands in the south. The warscore against you is starting to mount. Stannis or Renly should have declared war against Joffrey. It's usually Stannis. If you are lucky, the Tyrells will have gone over to Joffrey's side. Yes, that is another enemy. But, it's an enemy with smaller stacks. They too will usually be sieging your lands. After destroying the armies of the ironborn, it's probably time to head south. Be careful. You are looking for a smaller stack of troops to pick off. Don't let the big Lannister armies corner you. Just win a battle, relieve a siege or two and get the warscore against you down a bit.
The ironborn will land reinforcements in the north at some point. This will be far fewer troops than they originally had. Again, leave the south and go to confront them. Corner them and wipe them out again. Right around this time you should get a message that Stannis or Renly won the war against Joffrey. Because of the megawar system, Joffrey's allies were so busy sieging your lands that they completely ignored the Baratheon stack sieging King's Landing. Oops.
You should have a large enough warscore against the ironborn to white peace out of that war. Do so. You have no boats to go and finish them off so don't waste any more time with them. Because you have stalled against the south, you should still have a large amount of troops. It's now just you against the Baratheons. The other lords don't tend to help them. March on King's Landing. Many of the smaller vassals will join you, but it doesn't really matter if they do or not. You should have more troops then the other side at this point. Victory is only a matter of time. Watch out for Young Griff though. Stannis or Renly will surrender to him after a battle or two. If he wins, a certain someone in Essos will join his war and you might not want that. You should have more troops than him, so take them out when you see them.
Basically, Robb is easy. You just need to stall and wait for Joffrey to lose. Trying to win as Joffrey is much more difficult. Your allies have large stacks of troops, but they are content to siege random territories in the Riverlands instead of coming to relieve the inevitable siege of King's Landing. Stannis or Renly will have more troops than you, so there is really is not much that you can do.
