A couple of tricks I used to great effect with Germany.

1) Waiting for Britain to attack me when I just happen to be hanging around on a bridge. Britain destroys itself trying to attack me and is easily conquered after suffering major losses and levelling up my family members. This works against the Juli too.

2) Cavalry. Lots and lots of cavalry. Using a lot of family members early is a common tactic but against Gaul, Spain, and Julli barbarian cavalry is enough. Especially when you have almost a full stack of them. The more family members the better but 2 or 3 will do fine. Cruising through Spain with a full stack of cavalry is fun. Easy but fun.

3) Population control. Don't go crazy making those awesome German phalanx troops. They are great but they take 120 people. Also use slavery to boost a city or two so you can make gothic cavalry and berzerkers.

4) Don't buy everything you see. Show restraint when spending your money and grow those cities. Once you conquer some better cities your production will ramp up. You can take your time a bit and build up your economy and armies.

I found this faction to be easier than Bruti. The reason for this is because there are so many easy targets for you to attack, you don't have to worry about those marian reforms, and you don't have to worry about Egypt (the only enemy that has ever really given me any problems). You don't have the money of the Bruti but you don't really need it as much.