Well, same as posters above me said. And if you really want easy independence, I'd suggest waiting for the time Iron Throne has some kid reigning (or just someone that can't lead armies yet) and then revolting.'
For if king of Iron Throne can't lead armies, he would most probably be in King's Landing. (check this - character screen, one of the tabs on the right side should have info like 'reigning in X,Y' or 'tutored in X,Y') That means that if you win the assault/siege of this holding (King's Landing in this case), you'll most probably capture the king. And capturing the leader of entire enemy coalition sets your warscore to 100%. This tactics requires ability to amass army of certain magnitude, though - garnison of King's Landing is quite big, and if you siege it, you have almost certainity that after some time the allied armies will come to kick your arse.
You might feel it's like cheating, though.