You need to build a Hall of Heroes. Once it is complete, it comes automatically with the most basic Hero for your civilization unlocked. You will now need to unlock ANY 5 other heroes you want to in order to unlock this achievement. There are three heroes per civilization with different costs. You could unlock the other 5 basic Heroes to unlock this achievement. You could also unlock 4 other basic heroes and then 1 hero from a civilization whose basic hero you already posses. It really does not matter which heroes you decide to unlock as long as you have unlocked 6 in total in the Hall of Heroes.