With Title Update 5.2 came a revamp of the Arena mode and with it a new Arena Event similar to the Faction, Pet, Class, or Bounty events that appear in the Mini-Games menu. During these events you will need to earn 100 Valor to unlock the achievement.
The Arena has also been revamped and has removed the ability to use a Hero and now only takes 6 wins to complete instead of 8. Per the patch notes, completing a full run of 6 wins will net you 50 valor. So you would basically need to run through the Arena twice, winning all 6 times.
Valor is only available during the Event and can't be earned when the event is not running.
Per the Patch Notes
We have added a new event focused around the Arena.
During the event players will earn Valor based on the number of Wins from the Arena run:
0 Wins: 0 Valor
1 Win: 0 Valor
2 Wins: 5 Valor
3 Wins: 10 Valor
4 Wins: 18 Valor
5 Wins: 30 Valor
6 Wins: 50 Valor
The Mini Games Menu will have an Arena tracker (similar to Vault Event) which will track the amount of Valor a player has earned towards the current reward Stage.
The Arena Tracker has 5 different rewards that it will progress through before starting back at the first reward type again. Rewards that have been earned will be sent out via the In-Game Mail:
Reward 1: 3 Gem Keys
Reward 2: 1000 Jewel Shards
Reward 3: 2 Event Keys
Reward 4: 60 Writs
Reward 5: 1 Orb of Chaos
Arena of Valor also will support a pre-set Troop that will replace one of the regular troop draft picks.
The Troop will be given each time a player starts an Arena run during the Event.
The Troop is not limited by the normal rarities available in Arena (e.g. Common, Rare, Ultra-Rare, Epic).
These pre-set Troops may not happen in every Arena Event.
Any Arena runs started before the event begins will not give any Valor, and any Arena runs in progress at the end of the event can be completed like a normal Arena run once the event ends.
Valor doesn’t carry over to the next event, and any reward progress is lost once the event ends.