In order to complete the achievement Gurubashi Arena Grand Master, you must loot 12 chests from the Gurubashi Arena. Here’s a brief run-down of everything you need to know to get this achievement:
Every 12, 3, 6, and 9 o’clock (server time) the NPC Short John Mithril will announce that he has placed a chest in the middle of the arena. Once the chest appears, the first person to loot the chest will get an Arena Master trinket (and some other goods).
The Gurubashi Arena is a free for all PvP zone, so even if there are members of the same faction you can still engage in combat. During peak hours you may find yourself in a free for all competition for the chest. If you can get to the arena during off-peak hours, this will increase the chance that you don’t have to fight for it.
During peak hours, you’ll either need some good PvP skills, or some friends to help take care of other combatants. However, since it’s a free for all PvP area, you won’t be able to heal each other. Do this successfully 12 times and you’ve completed the achievement!