This achievement is for using an exclusive avatar or frame.
When you click your player's picture on the top left of the screen, it brings you to your player profile. Once this is open, you can click the "Edit" button underneath your picture. This is the area where you can select avatars and frames.
The avatars and frames are separated into three different categories. "Free", "Premium", and "Exclusive." The exclusive ones have an orange/yellow background.
Unfortunately, the only way to obtain the exclusive avatars or frames are either purchasing them with holographic shards or meeting the requirements for the ones with a lock over the image.
Holographic shards can only be earned in game through two methods - either you must pay real money to purchase them in an offer (offers are fairly random and change every day, and there is no guarantee that any given offer will provide holographic shards) or it is possible the trader may offer some sort of deal for shards in exchange for a free currency (I have not seen this offer myself, but per the comments it takes 5.2k medals to purchase 30 holographic shards).
The trader appears randomly every few days and goes away after a set period of time. The trader appears at the bottom left of your base, underneath the main power supply. The trader's offers are random, and you can pay 1,000 medals to refresh his inventory or 2,000 medals to refresh his inventory thereafter.
I have been playing this game pretty much daily since release and have only managed to see the trader come around maybe 15 times, and never noticed him ever offering holographic shards, so if you are wanting to obtain this achievement without paying any real money it could take a very long time.
The only other way I might recommend unlocking this is to meet the requirements of one of the locked exclusive avatars or frames. Currently there are several that say they can be purchased from the "Halloween trader" or are available as a reward from the "Spooky Carnival." These are both brand new features with the new update and don't appear to be active in the game yet. They are described in the patch notes here:https://forum.gameloft.com/viewtopic.php?f=2232&t=658320
Keep an eye out for these events going live and it may be easier to obtain this achievement.
[UPDATE] The events are now live, and the achievement isn't that hard during the event. There are a series of 6 events from Oct. 29th to November 4th. You need to achieve a total of 10 million points across all of the events in order to get the highest tier reward for the event, which is an exclusive frame. On the first event alone, I scored over 30 million points, earning me enough to get the frame without even needing to play the other days. In addition to this, there are pumpkins strewn all about the map, which if attacked by a commander will give you a coffin chest which can be opened for candy. Candy can be exchanged for an exclusive avatar at the trader, who seems to be sticking around pretty much through the entire event. It costs 4,000 candy for the exclusive avatar. The comments of this solution also has some good information including that one of the frames only costs 1,000 candy. After a day and a half of fairly heavy playing, I already a am very close to having 4,000 candies should I decide to purchase the avatar. In total, this is a very easy achievement to get over the course of the event. After Halloween ends, however, this could go back to being very difficult.