Interesting... in my game, the generals always march together whith the main force, is extremely hard to go and get them , only from behind.Archers doesnt seem to be to effective against romans. (version 1.5, medium & hard)
That's why I said use cavalry, and not infantry ;)
With heavy cav it's easy to go around the enemy army's flank, or break through a unit.
The archers aren't for killing the Roman unit, but to drop their morale (that's what fire arrows do).

Yes, they are... Howewer, when they face armored hoplits possitioned in a street (even the hoplit militia), they will get killed very easily... that is why roman infantryman, legions, hastati, principes, are, in my opinion, much better for sieges.
Are you using any mods or something?
Cataphracts break through phalanxes pretty easily.

Isn`t the roman infantry the best in the game ?
They are, but they're not so good that you need to use difficult tactics like yours ;)