True End

To get the true ending you need to just follow the main quest BUT do not approach the final boss. You need to clear the scene that happens right before it but not approach the final boss. Instead you go to the not so hidden area in the south east of the map by taking this rock bridge across the river. Then all you have to do is finish the new side quest followed by main quest.

Hidden Boss

After completing the True End you need to kill all the ghouls that spawned on the map. They are yellow difficulty mobs. They look about 3 feet tall and 70 on the ugly meter.

True End

Easy enough, all you have to do is complete this guy's second side quest that starts after you complete his first side quest and main quest.

Hidden Boss

This one depends a little bit on luck. You will need to start this guy's fishing side quest. During this side quest he will ask you to fish for a golden fish. But you can also find normal fish and garbage. You will need at least one of each to trigger the hidden boss after completing the main quest. Fishing ends as soon as you get a golden fish which is entirely luck based. So in some cases you can just be screwed and have to do it again in a future run.

After completing the main quest you will need to visit two locations.
^ This is the sea, where you need to throw in the dead stuff that humans hate. (garbage) ^
^ This is the abyss, where you need to throw in the living stuff that the dead need.(fish) ^

After doing both those things you can then fight the hidden boss where the narrator was standing.

True End

You need to start by getting a side quest from this guy in the Saphhira village. Make sure to complete this quest before progressing on the main quest.
After resuming progress on the main quest you will get an option to choose between rubra, sapphira or to ignore them. Ignore them. You will get the true end.

Hidden Boss

Before you complete the main quest, do the scholar's sidequest to find 4 relics. Also do the side quest that talks about an island in the middle of the lake. Once you complete that sidequest, you will have to wait a while until the lake freezes. Visit the middle of the lake and go through narrator's dialogue. After completing the main quest and the side quest that freezes the lake, you need to walk somewhere around these marked locations to get 4 relics.
After you do that, you need to place one on each of the four outer columns including the broken one.
Then the hidden boss should spawn in the middle.

True End

You need to complete the witch's side quest to get the true ending. There are multiple locations for water you can get for her quest. You need this one to succeed. Alternatively there's actually the pond below the third abandoned village on the far right of the map but that's further away.

Hidden Boss

To get the hidden boss to spawn you must make your way to this location. Midway the narrator well give you a choice and you must refuse and fight zombies along the way. Next complete the main quest and work your way back to the location.

True End

This one is a little tricky. First off, DO NOT DESTROY THIS MAGIC CIRCLE. (This will kill the npc with the sidequest you will need to complete the true ending.)
Next, find this friendly fella.
After you have an item that lets you open the sealed doors. Start by fetching the bottom room's sword piece BEFORE you do any trial from the main quest. Then don't complete the bottom trial for the main quest until after you collect the rest of the sword pieces and give it to the blacksmith. After you complete the side quest, go to the bottom trial room and lose the fight to the shade on purpose. Finish the main quest and you will get the true end.

Hidden Boss

Easy enough. Visit this location before doing the main quest and destroy the circle. After completing the main quest revisit the circle and you can do the trial again but with all 5 rooms and a hidden boss.
PS: Don't bother trying to do both the True End and Hidden Boss at the same time in this map. I tried it dun work. ):

True End

Pretty easy. Don't immediately enter through any of the gates in the middle of the map. You actually just have to go talk to the rosa corps. You'll have to wait 3 minutes then after the narrator will give you a choice. Choose to get the artifact alone. (You might need to complete the scholar's quest first.)

Hidden Boss

Complete the true ending first. Then return to the memorial sites and do them again. After just stand in the middle where the artifact was.

True End

You need to complete the true ending for Pond of Seclusion first. Once you start the main quest, you must find the witch and complete her side quest and the following quest she gives after that. Avoid returning to the camp and finishing the main quest until you finish all the side quests including both of the witch's quests first.

Hidden Boss

This one is actually pretty easy. Just wait for the 30 minute timer on the main quest to run out.
Just walk to this spot where the boss used to be and hidden boss will spawn.

Now fetch me a sandwhich. You might just get an achievement.