Always go for unrelenting force, like i said in my starting guide back in autumn. The most value you can get out of one army is if it is fast enough to get where you want it. Preferably it can hop from one settlement to the other in one turn.

I'd consider ambushes if i am ever on the back foot, i.e. I'm a gaul or german and the Roman war machine is methodically pushing inside my home provinces. I was expecting this to happen, but it hasn't so far. Sometimes I actually miss the cheating Roman AI from R1.