Successfully complete a chapter to unlock the hard difficulty setting for any mission in that chapter. Press L1 or R1 to toggle between chapters and then press Square to switch between the normal and hard difficulty for any mission.
Easy money and schematics
Select a good AC to start with then get to at least Chapter 3. Start fighting the sim ACs in the ACSIS menu. A good amount of money is given out per fight, and as a bonus you get the defeated AC's emblem and schematic. You can use the defeated AC schematics to defeat more ACs because you only need a buildable AC to start a mission, and not a sim. The only drawback is that you can only use AC schematics that can be tuned to a certain level. If you have enough points to tune the AC up then you can use it, otherwise it will be highlighted in red instead of yellow or white. You can also use schematics earned in the sim online.