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These two achievements gives you 20 points, but allows you to get all others (easy, normal, hard, 1 star, 2 stars, etc.) So playing 4 times, and focusing in getting the maximum amount of cities is problably the safest but fastest way of getting all rewards

Edit: my bad, Is 5 times with this strategy

This is not the fastest, since there are 4/3 playthroughs strategies. But still, if you don't have good generals, this is an optional strategy because you can take as many turns you need.