I found that the easiest way to get this achievement is to do the Squad Upgrades squad building challenges. In these you have to put a full 11 player squad in with at least 40 chemistry, and once you submit it will count for 1 challenge done. The best thing about these challenges is that they are repeatable, and also with them only requiring 40 chemistry it makes it possible to complete the challenge with players out of position, making it easier.
There is one challenge for bronze, one for silver and one for gold players, all of which are repeatable. If like me you have kept all your unwanted players in your club, you can put them to use here, if not, keep them from now onwards! To help you along if you don't have many players, there are bronze packs in the store for 400 coins and you can normally get 3-5 players in them.
To help you make the most of the players you've got, I have a couple of tips. I used the squad builder to make the squad (rather than starting from scratch which takes longer, except for the gold challenge as it will take most of the players in your main squad, which most people want to keep), and once it has built a squad for you, fill in any blank spaces with players either in the work area at the bottom of the screen or from your club. Then see what your chemistry level is and if it is high, try and replace some players with others that play in a position you have an abundance of. For example, I found I had a lot of bronze goalkeepers (as you only need 1 per time), so after I had built the squad I would replace the some outfield players (such as CAM or CDM, which I didn't have many of) with goalkeepers until I was on or just above 40 chemistry. This means you have these players for the next challenge, knowing you will be able to fill the position and not use an out of position player (which doesn't give you a lot of chemistry). Also, if you find you are just short on chemistry after it has built the squad, try swapping some players positions in the squad to gain the extra couple of chemistry points you need. Sometimes you can find moving a player out of position will actually increase chemistry because it creates an orange or green link, which gives you more chemistry.
If I have missed any information, or anything needs elaborating, please feel free to comment!