Recommended: Bring three friends level 60+ to help you open the Banishing Gates and have one of them as a Thief to increase the Garlaige Coffer Key drop rate.

Head to Garlaige Citadel and pick yourself up a Garlaige Coffer Key. I found the simplest way was to drop down to the lower floor and fight Over Weapons and Vault Weapons. After you get a coffer key, head back up a floor and pass through Banishing Gate #1. All coffer pops are behind Banishing Gate #2 or #3. I suggest you go past Banishing Gate #3 as there is no return once you pass through Banishing Gate #2. Search for the coffer there, and if you can't find it, look for it past Banishing Gate #2. Once you find it, open it up for your reward.