Nice to see you loaded up Rome 2 back again.
I mostly play with Rome, recreating the Roman Empire by using historic armies and lots of Aux troops from the region where the Legion is stationed. My core is Roman troops, but they only consist of 4-5 legionairies and a first cohort. When the legionairies gain a silver chevron, I recruit a veteran legionair (maximum of 2) into the army. When the legion is fighting in Gaul, I recruit the Germanic veteran instead. Also as Rome I only convert the main building and build aux camps where i want to recruit troops. The other buildings I leave them in the state I conquer it. When I fight battles in Gaul I enslave the captives (Julias Ceasar did this in his Gaul campaigns), all the others I release the troops. When fighting Carthage I kill the troops. And I will also raze Carthage to the ground.
When I play Gaulic (Averni) I mostly have armies with heavy infantry and cavs. They are very good. They also have a good agriculture/food production capabilities. Research is the main weakness in them due to lack of research buildings. Roleplaying wise, the averni were known as the most powerful tribal hegimony in Gaul. They had military dominance over their neighbours. I try to maintain good relations with the other celtic tribes and protect their borders as well against the germanic factions and/or the iberian factions (they tend to come north). Because of the military power i do not use ambushes. The goal here is to make a large Celtic Confederation before I head to Rome.
As a Germanic faction (Sweboz/Leugoz) I make use of ambushes (remember Teutoburg forest

). The main things I do with these Germanic tribes is mostly raiding my neighbors. I barely take the settlement if they only have 1 settlement left. If they have more I liberate if possible, otherwise I take it for my self. I tend to use lower tier spear units, bowmen, javelineers. I don't use any sword unit until i fought the Romans (Historically they mostly used swords from fallen Roman soldiers). When attacking, I use a wedge formation with my troops where my general is leading the spearhead. You can try to make only armies that are not more then 1k men strong because that's what they used when raiding. But I guess it's not possible to win battles that way. Most of the other tribes will field a larger army. If my general dies, the army goes with him and I start all over. Also I never use two armies close to each other (rivalry exists here).
As Carthage I use a hoplite heavy army or pikemen heavy army in Magna Gracia to conquer Rome and the Greek factions. I mostly use a sword based army in Iberia (sometimes I play with a hoplite heavy army) and always a sword army in Gaul (when I'm conquering Gaul, most of the times I just raid settlements if I'm in war). I try to maintain as much trades as possible.
I still have to find some good roleplaying ideas with the character traits you get when you recruit a general.