Grace Note said, in an answer I hadn't read until it was too late:
The best ending is achieved by following specific requirements at certain points in the game. It's very difficult to explain without any spoilers, but it involves skipping certain events and items to get better items at a later point, and making sure you follow through on an optional sequence of events.
Well, five minutes ago I got stuck for long enough in Cave Story+ to finally fire up a walkthrough for the game. I was in Labyrinth B, looking for a power source of sorts, and I thought I'd done everything there needs to be done in the general neightbourhood, zealously saving after each event. Then I read:
There are several ways to get to the door: [a door I'd already reached]
- If you have the Machine Gun on level 3, boost upwards by firing it down. [like I've been doing for ages now]
- Go ahead past Monster X, talk to Professor Booster, and get the Booster 0.8. [yeah, I've got that] This will cause you to lose your chance to get the Booster 2.0 later [what? why?], and also keep you from getting the best ending [OH GOD WHAT?], but it will allow you to use the Booster 0.8 to reach the door to the room with the Arms Barrier. [no, god! no, god, please, no! no! NO! NOOOOOOOOO-]
I had done that five minutes ago. Three hours of progress down the bin. He didn't even ask me if I wanted the stupid booster or not.
- Do I want to get the best ending, or is it something so incredibly hard (think "I Wanna Be The Guy!" difficulty) that's best left ignored pretending it doesn't exist?
- Is this path even available at all on Easy difficulty, what I'm playing on?
- What is it I must and, most importantly, must not do to get the best ending?
Best Answer
I will say up front that I don't think that Easy difficulty affects this, but I've only played the original. However, unless they felt like being callous, it shouldn't have an impact (considering you can access the Best Ending in other alternate game modes as well).
The Best Ending isn't incredibly difficult unless you (like me) are playing at 3 Health. If you've been picking up all the health increases and arm yourself properly, you should be able to accomplish it eventually. So I would recommend going for it because it is a nice ending. As well, even if you meet the requirements for the best ending, you can always opt to still take the normal ending (or even the bad ending).
You basically need two things to get the Best Ending. This is obviously a spoiler but you're literally asking for it, and a 90% spoiler post looks stupid. You need the Booster 2.0 and you need Curly. All the big points you need to make sure to not screw up are all in bold below.
Booster 2.0
Curly
With both Curly in tow and the Booster 2.0, you gain access to the Best Ending. This does two things you will want to be aware of, though.
First, it does indeed ramp up the difficulty of the Last Cave section. It ends up filled with more spikes, and all of your weapon experience levels are dropped to 1 at the start. It isn't a severe ramp in difficulty, though, just a taste of what is to come. This shouldn't worry you as you'll have the Booster 2.0, which is worth it, and the difficulty spike isn't that strong.
Second, after you beat the final boss, during your escape, you should pass by the house you had last used as a save point. This door would be closed if you failed the requirements. Inside will be another door that leads to the Sacred Grounds. This is an extra dungeon that you must clear, upon which you'll receive the Best Ending. You do get the option to use the Save Point again so you don't have to re-fight the final boss. As I mentioned earlier, though, this is optional and so if you want to you can always ignore the house and settle for the Normal Ending.