Don't bother with this on your initial playthrough - the easiest way to get S rank in combo is to use Maxus Drago's charged B attack, and you unlock him after clearing the game for the first time
Also if you leave a weak enemy or crystal alive at the end of the match you can stand by a hologram to heal fully for an S in finishing health
As for buildings remaining, with 2 Ss you can get a C and still get an S overall - otherwise using air attacks helps keep you from breaking too many
You can replay levels at any time in the Interspace