1.) Take advantage of the current hotel/car rental MQD offer for travel completed by October 31 and book by September 30. Book expensive hotel suites to get a lot of MQDs.
2.) Book and fly a lot of first class and Delta One fares.
3.) Charge it all on a Delta Reserve Amex

If Diamond is important to you, then a combination of these three things will help you get it in four months.