Flanking I works against non-archers, but Flanking II is what you want, as Kirkav said, it provides immunity to first strikes. I use flanking as "mounted siege", if a flanking unit survives the attack, even the "retreat" does (some) damage. First strikes obviously negate this damage, that's why Flanking II is so good. I usually do the following promotion method: I lead the units unpromoted until I reach the city. Once there, I promote units to flanking as long as there's undamaged defenders. Once units have been damaged, Combat I becomes more interesting, at least for 1 unit that will receive Medic I. With stables, I do the same, except I go for Flaking II and when defenders are softened I go Combat I + Shock if there's spears/pikes, or Combat II if archers are defending.