The other guide is written wonderful but if you are like me and didn't want to pay for the dlc you could play random or find a friend who bought the season pass or Morituri map pack.
Choose the map Wrath of Rome
. I do not know if this was random or it's the same Multiplayer maps as the Solo map False Gods. Need somebody to confirm.
on Round 1 after we completed the round then when it went to Round 2 I unlocked
at the end of Round 2.
So hopefully this works for everybody. If there are other random achievement unlocks let me know so we can make the solution better.
Also if you negative please state why to make the solution better.