Tips on how to earn medals?

How to spend medals - buy good gens. Don't use them to upgrade tech or exchange for resources (there may be some exceptions, when you need resources to buy medal wonder, but that you need to calculate first, whether it is worth for you). First gen to buy is Badoglio. If you don't have problems, you can save a bit more medals and buy Govorov first. If you buy Govorov first you can not to buy Badoglio. Than Guderian. Next one most people prefer Patton, I prefer Vatulin. After that you can build your 3 ECs. That's the way how to get 6 good gens spending less medals.

You can get medals from wonders and from clearing conquests and campaigns. It's better to build wonders first as they produce resources while you are doing anything else. First wonder is Great Pyramids - clear 1939 with Axis country in 65 turns. Japan is the easiest. Than goes City of Cuzco - clear 1939 for Allies country in 65 turns. For that you can play with France. These conquests are quite easy, all you need is to get lv 5 aircraft to get motorized infantry as paratroopers. Third medal wonder, Terracota Army can be unlocked completing missions, that you can't do without good gens. So just complete all missions you can and wait for it.

I was lucky enough to get a lot of wonder missions and got all 3 medal wonders after that I farmed for like 48 hours and got a few thousand