This achievement requires a lot of grinding. It requires you to spend coins and acquire every single improved weapon upgrade for every gun. It will probably be the last achievement you'll get in this game. I personally haven't unlocked this yet, but I will share my knowledge on farming coins.
Go around each location and kill enemies. They drop around 1-5 coins. This is a very small amount of coins, but you can be doing your mastery level 10 weapon achievements too while doing this method.
Loot supply crates. There are three types of crates which give you coins. There are wooden crates everywhere across the map. Simply break them and you will find a few coins inside them, around 1-5 as well. There are yellow and white crates. Opening these crates will reward you with 50 total coins. There are also yellow and black crates. These reward you with 100 coins. Crates refill every 24 hours.
Complete "Action Started" side quests. These quests ask you to do simple things in the map, such as planting a car bomb, or rescuing civilians. You will receive 200 coins upon completing these actions.
Go to Abby for your daily/weekly quests. You can usually get a few coins from completing daily missions and weekly missions. Some are really easy, such as double jumping 20 times.
You can also help other people with their missions. Completing their missions also grants you with the rewards. This includes experience, coins and ability points. I recommend this method if you are farming both coins and ability points for the "More human than human" achievement.
There's nothing really you can do except for these methods. Happy achievement hunting everyone!