Dreizehnn
if you have the idea of taking power of the iron islands, it is better after taking power develop the men and ships until the war of the 5 kings and after tywin is being defeated by robb stark and protecting in harrenhol far from his lands you can invade lannisport with part of your fleet as all the men of lannister lands will be accompanying your lord and there will be few defending your lands, you can even find the secret entrance created by tyrion to enter and take lannister rock and all as hostages to threaten tywin.