I recently obtained this so I wanted to share my experience to help others (keeping in mind achievers and twinks). Initially it was quite challenging because world pvp gets pretty intense here especially during the day, but I found a way around it to get this quite easily.

First off as you know the chest spawns every 3 hours (12, 3, 6, 9 server time). You'll find it's easier to open it during low population hours obviously, but almost always you'll have some kind of competition. So here's some important unknown facts to help you.

#1 - A second chest actually spawns. I've seen it mostly spawn a few minutes before or after the first, but I've seen it spawn exactly between the 3 hours time slot. So camp it and you might get a shot since it's rarely camped on off hours.

#2 - If you talk to the NPC he will stop. He will eventually drop the chest without even entering the arena if you keep doing this. This is good if someone from your same faction is camping the arena.

#3 - THE GREATEST TRICK OF ALL. You can see the chest while using "invisibility potion" which lasts 18 seconds. You can also use nitro boost with it active. So two things you can do is hide in an archway in the upper arena and boost down there OR if someone kills you just rez, invisibility and loot. Using this method I've robbed the trinket under the nose of over a dozen players. Got ten trinkets in a row and all of them sitting there puzzled. FYI though the lesser potion requires level 23 so sorry level 19 twinks.