This achievement requires your guild to spend 15.75 million in one week. If you're able to luck your way into an active guild, great. If, like me, you were focused more on the achievements than trophies then you will likely end up with this as your last remaining achievement.
My guild clearly wasn't going to achieve this (without me contributing 15.5 million), so it was time to find a new guild. The forums on TA for guild recruitment were pretty dead, but the Gems of War forums had lots of posts for top tier Guilds looking for members. I found this thread:
http://community.gemsofwar.com/t/29-30-gem-gem-assassins-dag...I contacted the Guild Leader GamerTag: "Mrs Affects" and asked if she would be willing to let me join their Guild to pick up the achievement if I would donate a ton of gold. I had ~550k before I worked on the PvP Tier 1 Achievement and ~760k gold after I was done with it. She offered to let me join and get the achievement if would donate 1 million gold. Normally that would still take a fair amount of time, but I had banked up a lot of gem currency, and since I was done with the game I wouldn't need it anymore, so I spent ~500 gems to get me to 1 million gold. They had already completed all the tasks for the week (it resets on Monday, and they already had it done by Wednesday), so the achievement popped immediately on me joining. In exchange I gladly donated the 1 million gold towards an "Epic Task" which rewards their guild members with some bonus keys. I then quit the guild (I was in the guild less than 15 minutes).
She seemed to be very happy with our exchange, and said she'd be willing to do the same for others, so if anyone else has only this achievement left I recommend contacting her.