Cave Story/Sacred Ground

You have chosen to aim for the toughest third ending. This will be extremely difficult. If you have picked up the timer, it will show you your current total time, you can try to complete the whole section of the game in under 6 minutes for a special surprise. There is not a single save point during this part, so you have to be very careful about your health, avoid getting hit, collect hearts dropped by enemies as well as the laid-out ones, and conserve your healing potion for as long as possible. If at any point during this path you die, you will have to restart the entire level.

Sacred Ground B1
You must navigate a seemingly impossible set of spiked traps. To begin with, run to the left, quickly tap Z three times while holding left to bypass the first set of deathtraps without wasting Booster fuel. Then hold right, proceed to tap once or twice and then up and tap. If you hold down Z you will run out of fuel and die. Expect to master this part as you will be doing it often.

After that point you must jump down to a ledge, proceed left, and use the tapping to conserve fuel, after a long drop you should be hugging the right side of the column through which you are a falling, so that you can Booster onto the ledge, which holds a +5 HP health capsule.

Proceed to tap fly all the way to the door, and near it pick up the unconscious Curly with your tow rope. Then proceed to the door.

Sacred Ground B2
With unconscious Curly strapped your back, you must navigate through this area with the least amount of damage (and time). Butes(flying angels) will assault you in droves, doing 5 HP damage every time they hit you, and boulders will be falling from the ceiling doing 10 HP damage. Avoid the boulders, kill any angel that gets in your way, and proceed to the next area.

Sacred Ground Unnamed
You will then enter an unnamed part of the Sacred Ground. At this point Curly wakes up, and she will fire a Nemesis gun in the opposite direction you are firing your own, unless you are standing on solid ground and aiming up, in which case both shots will fly up. She is using her own Nemesis gun, so you can use yours and have a double Nemesis shot.

Go left while preserving your health. You will have to blow up certain bombs that block your path. The bombs' explosion radii are rather small, so you do not have to run far, and their damage to you if caught in the explosion is survivable. After the wall with 3 bombs you will find a single destroyable block. After killing all the enemies, hit it to reveal a chest with a Missile Powerup +24. (If you don't have any Rocket Launcher, instead you will obtain a normal one with 100 missiles in the clip.) When you finally reach the leftmost part you will be fighting the Heavy Press boss.

If you stand firm on the ground and point up, both you and Curly will shoot up at once, allowing you to do damage rather quickly. When the Heavy Press is dead, get out of the way, as it is going to fall down and crush the floor and kill you if it hits you. Follow through the hole it broke.

Passage?
You will find yourself at a previously visible but unreachable part of the corridor that connected the Plantation with the room of the Statue Carver. The left door leads to the Plantation and is blocked, the top right door leads to the statues. The carver is gone, and you can blow up the statues for power ups such as energy (large) or multiple hearts. Proceed to the door on the bottom right to reach a corridor leading to the final boss of the game.

Final Boss
Proceed through the second corridor, speak to the dog, proceed through the door on the right to reach Ballos.

Give Up?
If you choose to give up with the Sacred Grounds, you will notice that the door you entered in is locked. Simply go to the bookshelf and inspect it. It will state that it can "turn back time a minute or two". If you do give up, answer yes to its question and you'll be teleported back to the moment when you came out from the throne room. If not, say no.