Unmissable if you simply keep playing. Every game you play you will get an amount of xp and gold at the end. More for wins but still a good amount for losses as well. You get a higher payout for ranked online wins than you do for solo campaign wins but the early solo matches do not last nearly as long and you can do 2-3 in the time it could take to do one online match. If you enjoy the game and ignore this achievement and simply play for fun it will come in time.
Note: the longer a match lasts, the more gold and xp you will earn. the game has a multiplier tied to game length and if you surrender and loose the match rather than just be defeated, you will loose this multiplier and gain less gold and xp.