Seeing that the Sylphs gifted you a crystal of light, Ramuh agrees to test you before passing judgement on mankind.
How to Unlock
The Striking Tree (Hard) trial is unlocked by the level 50 Main Scenario Quest Levin an Impression.
Random players will get a circular lightning outline around them while Ramuh casts Thunderstorm. This can be used to your advantage when Ramuh casts Chaotic Strike, which inflicts the Fear status on two random players. The players with this status will slowly walk toward Ramuh, who will soon cast Thunderspark, a circle attack centered on him. Other players can remove the Fear status by hitting the affected players with their Thunderstorm attack, which is indicated by a circle of lightning around them. Be sure to look at the party list to see who has the Fear status, so you can help remove it from other players.
Ramuh will place an increasing number of lightning orbs around the area. These orbs can be absorbed by players, but each player should only collect exactly three orbs. This will give the player a Surge Protection buff that reduces damage.
Ramuh can tether two players together with a line of lightning. The two players who are tethered will take damage every time they use an action. Players can remove the tether by absorbing three lightning orbs as mentioned above.
Eventually, three Grey Arbiters will appear around the area. They can cast line attacks that are marked by a line of lightning. Kill the Grey Arbiters before Ramuh finishes casting Judgement Bolt or you will not survive the battle.
Judgement Bolt is Ramuh's ultimate attack. It inflicts a large amount of damage to all players. Its damage increases if any Grey Arbiters are still alive when Ramuh casts it.
More Grey Arbiters can appear later. They will cast line attacks as before. Kill them to reduce damage to the party.
Completing the Quest
Defeating Ramuh will complete one of the quest objectives for the Main Scenario Quest Levin an Impression.