Gangs of Sherwood

Challenges can be attempted at your base, Major Oak. There are 8 in total. Go to the training room, and head to the left and interact with the green console. Here, you will see the list of 8 challenges. They are:

Against the Clock
Defeat all the enemies before the timer runs out (120s).

Executioner's Training
Perform 2 finishers (have at least 2 Instinct bars, and then press + at the same time when an enemy has low health) before the timer runs out (100s).

The Juggler
Keep an enemy in the air for 8s. Personally, I think this one is the trickiest. My approach was to use Robin, get the enemy up in the air, and stand directly underneath them and use light attacks only. Don't press quickly 4 times in a row, though. Only press it 2 - 3 times. The enemy will likely hit the floor, but that is ok as the timer should still run for a second or 2 even after they've hit the ground.

Death and Taxes
Defeat the Tax Collector before he escapes.

Air Supremacy
Defeat all the enemies. The trick here is that they will only take damage if you launch them in the air, then attack them while they are still in the air.

Perfectionist
Defeat all the enemies without being hit.

Performance Artist
Reach and maintain the S-rank until the timer runs out (90s). This is the other difficult one. I personally find it easier to use Marian with her magnetic dagger abilities. My approach was to get Rebel Instinct, activate it to give everyone lots of daggers. Then aim and press to throw a dagger the causes a Flux triggers. This will deal a lot of damage, and raise the combat ranking by a lot. Still, use whatever character you're most comfortable with.

Obsessive Compulsive
Kill the 3 enemies in order, before the timer runs out (30s).

Good luck!