In war its hardly single unit vs single unit. However I have had promachoi defeat multiple units of Polybians and slow down a couple early praetorians. The trick is to out manouver them with armies I defeat the Romans when I am outnumbered Strategically and Locally on the battlefield by out manouvering them and using Hannibalesque tactics.
@Roarer
That may be however he never ruled it out and he is thinking of using Bastarne and art and he was just asking for tips.