Hi guys,
I've played all three factions at different levels frpm medium to hard. here is what I found. Fight the romans as they factually did. This works better the harder the level you play and also works better the more experience your troops are.
My standard Roman army is this - five units of Hastati in the front line - gaps between them wide enough to pass another cohort through. Second line Principes staggered to the gaps in the first line - like the black and white squares on a chess board. if you have more principes, extend the second line out beyond the first. This gives good cover against flank attack because the AI will usually attack the open end of the first line. You simply swing the end cohorts in the second line outwards towards the flanking attack. The real benefit of fighting like this is it stops the enemy from cavalry and/or infantry rushing your centre formations and getting at your misslie troops in the rear. I've had armies hit by multiple heavy cavalry units in the centre { which the AI does if it's cavalry heavy in it's army}. While I've taken losses The enemy cav has been stopped and I move my nearest second line cohort(s) forward to butcher the now milling cav. Against large Ai infantry units - like Gaul warband this also works real well. The much larger warband will lap around my front cohort and in doing presents its flank or flanks to the counter-charge of my second line cohort. My missile troops - usually two each of archers and skirmishers stay healthy ( just remember to give them targets to fire on that are not the enemy units now attacking your front line cohorts} The remainder of my my basic Roman army is made up of cavalry and heavy weapons like ballistae.

Another good idea is to have another smaller army following behind your main army. This smaller force has my seige weapons and at least a couple of replacements units to dribble into my main army to fill out losses. Just remember not to let it get too far away from the support of your main army. If the enemy appears in numbers have your Army commander build a fort. He then moves the whole main army of to fight while your siege/ resupply army is safe and sound holed up in the fort. Your main army then has the option of turning around and taking the beseiging force in the rear. This use of supporting armies works really well in places like Numidea and the open deserts where you are along way from cities and fresh supplies. Try it and see what you think.

Hope this helps.