Incompetent Legate sends 9,000 legionnaires to die AKA I need help with the focus tree goals

I've been playing the mod as Caesar's Legion, and I can't defeat the Hangdogs as they have this god-saving river of theirs. What should I do? And how can I move my navy in said river?

I've got some expectations on how to proceed: let them waste manpower at start, then push when I can, all the while having bombers harassing them.

Is it a good strat?