Buildings over Level 10 are created by combining 2 or more Level 10 buildings.
2 Level 10 = 1 Level 11, 3 Level 10 = 1 Level 12, ... For this achievement you need 10 different ones total. It doesn't have to be in one game.
It's important to notice that you can not combine buildings over Level 10, i.e. you can't combine two Level 11 to make a Level 12. Once created they are stuck on the playing field.
I wrote down all the Levels and number of Level 10 buildings needed and wrote down what I needed. My strategy was to aim for the highest number I could do while simultaneously work on unlocking characters (go for 2 Level 12 etc).