This is the finale of a series of three trophies. The previous two are:
This trophy is going to be a bit trickier to get, as there are two things that will end up causing some trouble. For that reason, I will go into depth with each Cosmosphere the order of events you want to go through (credit to Krystal109 of GameFaqs for the hard work put into their extensive walkthrough). Whenever it says Optional, it would probably help to just go ahead and do it anyway, for the sake of the 100% CS completion.
**: Do not access this Cosmosphere unless you want Saki's endings. If you access it, even accidentally, you will lock yourself out of getting Cocona's ending or possibly Tyria's endings.