You almost can't even begin to discuss a question like this without specifying the degree of difficulty you're using.

On VH, it doesn't take much for your line to collapse. I've lost whole armies of decent Roman troops to barbarian hordes even when not outnumbered or outflanked. An 8-star enemy general versus a captain, and you got problems.

The mix of armies you have and they have is important too. Off the top of my head, the only thing special about the Spanish the Bull Warriors. They're probably going to beat more roman units straight up than any other barbarian infantry. The Spanish seem quite fond of javelins as well, but if you've mixed your troops that shouldn't be a big issue. At least you don't have to worry about snow.