This requires all missions S-ranked on both Normal and Hard mode. From the mission select screen, after you have completed the game on Normal you can replay any mission, in any order, even on Hard mode. So you could jump to the very last mission, or the ones that are giving you the most trouble, and get them out of the way first when tackling Hard so you can ease into cruise control.
Even while playing through the game, you can go back to previous chapters you've completed via the LB button and replay those missions if you like before completing the game, but it's much easier to keep track afterward because you'll have all the missions laid out in tile format with your highest ranking on each one noted on its tile.
Hit "X" on the mission select screen to toggle between Normal and Hard.
I highly advise experimenting. The primary weapons you will be using are the EN-200 laser rifles on your arms, and either OGOTO or SIRIUS on your shoulders. However, for some missions where other sites advised me to rush through with OGOTOs I found it better to clear all the small stuff with my EN-200's first then switch to OGOTOs for only the main/big enemy. Also, when fighting fast-moving NEXTs, it's often easier to land hits with the SIRIUS cannons since they're nearly instant time-to-target whereas the OGOTOs take some time for the shot to arrive, allowing the enemy NEXT to dodge it.
Similarly, different missions have different requirements. Icebreaker Defense, for example, is strictly time based (and damage to the ship you're protecting too, but in order to complete it fast enough you by default prevent all damage). Missions where you fight enemy NEXTs are often time and damage-taken based, so you need speed while at the same time avoiding as much incoming fire as possible.
Tweak your tuning to apply to the mission you're tackling. Put points into Quick Thrust, Lock Speed, and Radar Refresh when fighting fast moving NEXTs. Quick Thrust and Lock Speed are good universally, as are EN Output and EN Capacity. Radar Refresh is really only useful against those fast moving NEXTs though, so put those points elsewhere for missions where the primary targets are slow or stationary. You can even put a few points into LOAD so that you can use smaller, more agile legs which would otherwise not give you enough LOAD capacity for your weapons' weight.
One example: on Internal Purge on Hard, another site suggests clearing the entire level with OGOTOs. I used EN-200's to clear everything except the 3 "large weapons" and the enemy NEXT at the end. For those two tasks I used SIRIUS cannons not OGOTOs, because he kept dodging my OGOTO shots. I found myself using the EN-200s to take out far more enemies than the guide on X360A suggests as they're so much more efficient and don't damage your own mech in close quarters. Also for any mission where I found that I did best with either OGOTOs or SIRIUS cannons, and wasn't going to use my EN-200s at all, I removed them to save weight and increase my agility.
Faster is better. I didn't use a slow, heavy mech for ANY mission's S-rank. The extra armor points are worthless if you're too slow to dodge much or complete the mission fast.