This is quite straight-forward once you have a medium tank. British, Japanese, and US all have a Tier 2 and Tier 3 Medium tank. The other nations are sketchy on medium tanks until higher Tiers. Medium tanks have a diamond with one line through it by their Tier number.
Just start a game with your medium tank, if you get destroyed you can just go straight to your garage and pick another ready medium tank and play another match! It will take some time to get the followup achievement for 100 matches played, but don't spend too much time watching games finish after you're out and it should go faster: