Whey! Here is where I jump in!
I read the Post-Marius guide upon the main website and have been using the formation for online play since, winning almost all time time.
In short it consists of 10 REQUIRED units and 10 of your own.
The 10 required are:
9 Urban Cohorts
1 First Legionnairy Cohort.
I however use 6 Urban Cohorts and 4 First Legionnairy cohorts in order to get a giant range of moral, also they seem to fight better... More troops too. (Urban = U, First = F)
UFUFU
Arange them as tight as possible so there are no gaps in your front line.
I would suggest then 3 units of Archer Auxillia (A):
UFUFU
.AAA
UFUFU
So thats 13 units, 7 left: Let's add four siege units (Pref. Onager) (o) for click-and-siege as well as mass murder at long range:
UFUFU
.AAA
UFUFU
0000 (Obviously more centred)
3 more left... 2 Praetorian Cavalry (P) and 1 General (G) for routing and shocking.
.UFUFU
C.AAA.C
.UFUFU
.0000
...G
This is mainly a defensive strat and can be achieved within 18,000 denarii.
The basic idea is that the archers are protected from the flanks, rear and front by a wall of Cohorts. If a unit of the front row of cohorts routes or dies, walk the same cohort unit deployed right behind it in it's place as a reserve.
This is the general idea of the strat and if it works excellent on Multiplayer it will rock on Very Hard in a campaign.
Don't forget to set your archers and onagers to fire, if the onagers aren't hitting cavalry, and set all your cohorts on fire at will so that you have a pila storm (Light Auxilla effectively) before the enemy hits you!
EXCELLENT Post-Marius tactic that has NOT failed me yet!
And here is the official link: http://rtw.heavengames.com/rtw/strategy/tactics/2005/02/02/historical_roman_legions_part_ii/