with regards to fighting phalanxes in-battle, it's a pretty difficult thing to do. the old, "pin em down and flank and fire" tactic is the most obvious thing, but it can obviously get extremely difficult, especially with the wide phalanxes outright blocking troop movements on the field. in my experience against the AS it takes a lot of all round heavy units that can stand up to their sarissas in a pitched battle. i generally engage with my heaviest (defense wise) troops and do my best to encircle the enemy, making sure not to outstretch myself and to engage any aforementioned "blocking units" if i run into them. expect heavy casualties though with this method, so i suggest seeing if someone else can help with that

likewise ill throw in some advice for the actual campaign. first off, dont forget to recruit levies as well, since they, whilst (generally) not being up to par with your roman troops, are likely to be easily retrained and, as such, might be your best bet as fort garrisons. you could always do your best with your legions, and if they achieve their objectives or need retraining, send a levy army in to take their place whilst the armies head back to rome.

as for any other ideas for the campaign, and im pretty sure others would suggest or at least think this, you can always send an army by sea to overtake a poorly defended settlement, destroy everything and pull out before an army can crush your own, and make sure to try and redo it over and over to hurt their finances and send their forces into confusion. after all, one more army chasing your raiders means one less army to take on back at the front. hope this helps and good luck