Out of all the barbarian factions, the Germans are probably the easiest to use, & certainly have the most powerful range of units of any barbarians.
Their spear warbands are VERY solid basic troops & their various axemen, chosen archers & cavalry are all good in their own right. Germania's main weaknesses are: small towns/smallish populations & quite low income.
The small population problem can be fixed somewhat by capturing Alesia, Mediolanium & Patavium from the Gauls. These towns have fast growth & large size potential.
The cash problem can be fixed by selling map info to EVERYONE. Everytime you capture a new town, go back & sell map info to everyone again - they will pay the same amount every time your map info changes. The romans don't pay well at all, but the Greeks, macedonians & carthaginians are all fairly generous. You can often make several thousand $ in map info for each town you capture.
As for field battles, the romans have the chief advantage of higher morale than other factions, so use screeching women to lower their morale. Also, later on, grab berserkers & nightraiders for their terror traits.
I also quite like the light barbarian cavalry for their low cost & very low maintenence & significant hitting power when you stack multiple units on top of each other - they have easily enough power to insta-rout most roman infantry up to the Marius reforms.
GM
Stupid questions & their appropriate responses from an Australian tourism web site:
Q: Which direction is North in Australia? (USA)
A: Face south and then turn 180 degrees. Contact us when you get here and we'll send the rest of the directions.