Ok heres my pre-marius formation in SPQR, (i only use it against gaul or other barbs)
"THE RUBBER BRICK"
C V_______V C
My principes in shield wall, whith hastati support, pins the enemy down while velites pepper them and lowers their morale, and triarii guards the flanks. And my cavalry fights of possible enemy cavalries flanking maneuvers, and when that is done i bring my cavalry around the flánks and attacks the enemy in his back. And now while surrounded, gauls break almost at once.