I'm not sure if I say something you know already, but the units are only balanced if you play with medium battle difficulty. Otherwise, the AI units will get bonuses that make them fight better than they should.

In any case, the most important things (that I can help you with, anyway) have already been covered, and it was a long time since I played as the Lusotannan so I can't help much more with the specifics. You can always try ambushing your enemy, though; your units - and especially the AP-armed infantry named above - are well suited for that, and it's a tactic the historical Iberian tribes were very fond of. It's very effective in-game as well.