If you are hunting for the “use only auto-resolve” achievement, I managed to beat the short campaign in 15 real-life minutes using this method. While you can choose to build up your economy and go for a slower conquest, simply pumping out Hastati in every single city and sending them to the front line against Gaul will crush them entirely in short order. That said, despite this very easy and quick path to late game domination, you don’t need to wait that long to destroy Gaul utterly. After capturing Patavium, start stacking growth buildings on it – this city has a hefty growth modifier and will likely be your first huge city – triggering the Marian Reforms very early on. You should be able to take both of these cities with little actual struggle – your Hastati are superior to their Warbands and the difference between the quality of your infantry will only grow as the game progresses. Take the small rebel town to your northwest in your opening turns and siphon off troops from the garrisons of your cities to move north towards Patavium and Mediolanum. One of the easiest factions to start with. Part 1: Julii, Brutii, Scipii, Macedonia, Greek Cities, Egypt
