-You must see an event with CC Sadist on the 10th floor of the Mugen Field before you clear Episode 7.
-You must see an event with CC Bipolar on the 10th floor of the Mugen Field during Episode 7, after Dees joins as just a head. You must ALSO see the first event in Crimson World to see this.
-Other events can be seen in the Mugen Field after each episode's start by diving down to the 60th floor of the Mugen Field in each chapter. You should try to get there at least once an episode, ideally early, just to be on the safe side. Consider diving repeatedly to see every scene possible as soon as you can.
-Getting to the 60th floor is required to see all events. You don't need to get all the way to the 100th floor.
-Some of the events are restricted by your Charm Level. You will need a minimum of 15% to see all of the events.
2) How much you've charmed the worlds may or may not be relevant - get all the star points on the worlds as early as you can, just to be certain. You'll sometimes need weapon drops and the like, and those are a pain to find, so try to keep a few units of each item. I should be able to provide a list of exactly which items are needed once the English version is out.
3) After you've seen all the scenes (and, I think, started Episode 13), you should have an event in the G Castle where you'll have to fight the Sun Goddess. If you never see this fight, you did something wrong.
4) Fight through the remaining enemies. When you battle against Ace, you can see whether or not you fulfilled all requirements - he should be Level 125 if you're in the True Route, or 95 if you're not.
5) Head to the last world, beat the final boss, etc. Fairly straightforward. It's not actually that hard to get to the true end as long as you make sure to see all the right scenes.