Flynn: Son of Crimson

What are Challenge rooms?
Challenge rooms are rooms accessed through locked doors found in levels with multiple exits. Inside these challenge rooms you will fight off waves of enemies. To enter a challenge room you must first acquire the key that corresponds to that level's door. (You can check which keys you have in your inventory)
Keys are found throughout the game, most of the time you’ll have to talk to an NPC and perform a small task for them, upon completion they will give you a key!
Tips:
A truly lifesaving option is to turn on “No player damage” in the accessibility menu. This makes it so our character is invincible, it's a bit cheesy but it makes the challenges literally unfailable. Note that you can use this option throughout the entirety of the game and it WILL NOT VOID ACHIEVEMENTS!!! :D

It is also important to note that you must complete all 6 challenge rooms in order to unlock the ”101% complete” achievement, as the challenge rooms themselves count as levels. With this in mind I would recommend beating the game first and going for this achievement, as well as “101% complete” when mopping up, rather than during your playthrough.

Key & Door locations:

Wildedge Retreat Challenge Key: The key to this challenge area can be found in Sanctia. Load into Sanctia and drop underground. From there just run left through the dark wall and that's your 1st key found!

Wildedge Retreat Challenge Door:
Load into Wildedge Retreat. Run through the level as normal until you meet Ceal, he will tell you about ranged combat. After you listen to him, move onto the next room. From there, instead of going up like you normally would, simply continue right until you reach the door.

Hollows Isles Challenge Key:
Load into Bleakstorm Ridge and speak with a fisherman named Rickathon. After you speak with him he will go to Hollows Isles. Now we have to load into Hollows Isles and speak with Jungo. After you speak with him, he will give you your 2nd key!

Hollows Isles Challenge Door:
Load into Hollows Isles and run to the right, the door is almost unmissable.

Clearcrest Springs Challenge Key:
Load into Clearcrest Springs, run through the level and at the end talk to Tholmin Jr. After his dialogue, load into Woodshade Settlement and speak with Tholmin. After his dialogue he will give you your 3rd key!

Clearcrest Challenge Door: Load into Clearcrest Springs and run to the right, the door is almost unmissable.

Crystal Grotto Challenge Key:
After you complete the quest to save King Neptyn at the end of Palace of Aquaras, we can load into Neptyn Capital and speak with Remi and he will give you your 4th key!

Crystal Grotto Challenge Door:
Load into Crystal Grotto and continue until you reach the room where the doors auto-lock and a wave of enemies spawns. After killing all enemies the doors will unlock and you can proceed to the right, be sure to drop down at the first chance you get. The door will be on the left after the drop.

Cedarstone Cove Challenge key:
Load into Typhir Deadlands (To access the Typhir Deadlands we first must defeat the 4th boss; Xellios “The Toxic Marksman”) after beating the boss, load into Typhir Marsh and go left and climb up the wall to unlock Typhir Deadlands. Once inside, continue right until you enter an altar, where you will find your 5th key!

Cedarstone Cove Challenge Door:
Load into Cedarstone Cove and complete the first area. The second area is a long stretch of spike platform. In the very first section of room 2, right as you enter the second area there will be a shaft in the ceiling directly above you with a small wood platform on the left. Simply mount Dex and jump up to the wood platform and dismount Dex as soon as your character model is on it. From there just hit the red block that's up there and it will unlock the metal door at the end of the level to access the challenge door! Just run to the right and keep to the lower platforms.

The Grand Archives Challenge Key:
Load into Zealock's Laboratory, progress through the level, after the area with the enemies in the vats. You will come to a dark area and you will see a set of glowing eyes behind the middle third of a stone wall. Click on the eyes and a guy will sell you your final key for 2000 gems!

The Grand Archives Challenge Door:
Load into The Grand Archives and climb up the first available ladder and continue right for your final challenge door!