Here's what worked for me:

1. Spend all the starting money on construction (palisade, roads) and new units (queue them up).

2. Capture every independent city you can until you run out of soldiers. Immediately hike taxes in every newly captured city as high as you can without killing population growth or causing revolt. Do not EVER exterminate cities - you're killing your tax base if you do. Remember that every new citizen of your growing empire means more tax money! Make a point to construct buildings that boost population growth, too.

3. Sign peace treaties and trade agreements with everyone you can. They'll attack you later anyway, but in the meantime you want to make as much money as possible and capture as many Eleutheroi cities as possible without getting into a war with another faction. One exception: Byzantion. Pontus will declare war on you when you attack it, but in both of my Getai games they agreed to peace immediately afterwards (I had my diplomat ready and waiting). Byzantion is nice, good trade income and you'll get some cool new units.

You should be out of debt sometime in 270 BC, and it should never get worse than around 12,000 mnai (VH/M). By around 260 BC, you should have an empire stretching from Bosphorus to the Alps. Sarmiszegethusa is a jewel - rich, big city with good unit mix. Move your capital there to minimise the distance-from-capital penalties.

You should be able to complete the campaign goals around 250 BC if all goes well.