That would depend on your timing and resources. Since the question is about saving the most money then I will go with applying for a Chase Sapphire Reserve credit card in branch (offer ends March 12) for the 100,000 UR point bonus, that comes with a $300 annual (calendar year) travel statement credit. The 100,00 UR points are worth $1500, the travel credits between now and next January will be $600 and you can downgrade the card to a no fee one when the AF hits next March. The annual fee is $450. So, that gives you $2,100 - $450 AF for a net $1,650 savings towards the cruise just for getting the card and meeting the requirements to get the bonus. Purchase the cruise through the UR portal.
You can also purchase 5% discounted Disney gift cards and fund a DVA (Disney Vacation Savings Account) with them. After 120 days of having the account open you pay for the cruise with the DVA and earn an additional $20 gift card per $1,000. Together that is a 7% savings.