just line up your units out of range of their siege weapons and send in one at a time heavily armoured high attack troops, unit you can get a break in the enemy, then try to get a unit around to get the missile troops and siege weapons. Other than that, just don't attack over rivers or, or hope it's a two bridge map because the ai is terrible at defending both. As a hint, on the strat map, highlight a province, right click then place the cursor over another adjacent province and you are told what terrain to expect to fight in, if you don't have a superior force, don't take the risk of attacking where theres a chance of a river.