Young Justice Legacy

Completing the YJ-challenges with an “A” grade (IMO) might be the toughest achievements in the game and will probably require a few retries, but are not too difficult. There are six challenges: Horde I, Countdown I, Survivor I, Countdown II, Survivor II, and Horde II in that order. They unlock with the natural progression of the game. As I understand the game, obtaining a grade “A” on each challenge is based on the number of enemies “you” defeat (not the A.I.) and the combos. You are graded on both just like the in-game missions.

Challenges cannot be played co-op, only single player.
Pick a healer that you prefer, Miss Martian or Zatanna.
Use your hero or team boost as much as possible.
On horde, just take your time and rack up kills.
On countdown, ranged attacks and faster players are better.
On survivor, I had both healers, stood in Miss Martian’s circle for all my attacks.

Horde challenges are each 10 waves with enemies progressively getting stronger with each wave. Simply pick a decent team (preferably with a healer, I preferred Zatanna) and defeat each wave. If the A.I.s begin to rack up kills, you may want to let them “die” (actually just in need of reviving) or check my edit strategy below. This will allow you to focus on gaining the necessary kills and combos for grade “A”.

Countdown challenges are each 5 waves with enemies progressively becoming tougher per wave. These challenges are timed. Each wave begins with 60 seconds on the clock and more time gets added with enemy defeats. Health does not regenerate at any time during the challenge, so having a healer on your team is a big help when you start taking damage. Just complete the challenge with a high number of enemy kills, combos, and time remaining on the clock to receive an “A” grade.

Survivor challenges are also 5 waves each and as with the other challenges enemies get stronger with each wave. In survivor, you lose some health over time, but you can regain health by defeating enemies. Playing with a healer is most important with this type of challenge.

Edit: I kept receiving “B” grades on many of the challenges, so I tried a different strategy and it worked to obtain an “A” grade on all of the challenges. I lowered the combat stats of my A.I. hero team (all the way down) with the exception of the healer’s healing ability (kept this stat to the max.). The A.I. members had fewer kills and I added to my numbers to receive “A” grades.

Good luck to everyone!

Edit: Submitted by Sparkz1977:
"It seems that, while you can't directly choose the difficulty of the challenges, it IS tied to the difficulty your campaign is set to!"