The strategy I used in my current campaign (VH/H) was to settle in Constantinople and the Balkan belongings of the Eastern Empire, then tech up and refit troops in Constantinople. In the meantime, the Goths and Ostrogoths got into horde and crossed into Italy, taking the Western Empire off my back.

So now I am expanding East, pummeling the Eastern Empire, with Antioch being my new eastern base. I am assembling a western army to cross the Adriatic and take Tarentum and Rome, avoiding the Goths hordes in North Italy. This will easily give me the 15 provinces. Admittedly, not a historical approach.

The huns apparently cannot build ports, but can use pre-existing ones. I am using the ones in Constantinople and Athens for my fleets.