One of the first challenges we face with this battle is hard data. Whilst Plutarch wrote briefly about the conflict, little detail other than losses was shared. So we need to rely on what the topograpghy tells us and how cohorts of Legions fought in this time.
A Cohort is comprised of men with individual frontages, 3′ per man seems to be the default figure for most armies and situations, and looks reliable enough for Roman armies. A useful rule of thumb for legionary frontages is 200 yards per legion.
The Post Marian Era still used troops in three lines, so each legion’s cohorts were probably arranged 3-3-4, with the 4 in the third line, or conceivably 4-4-2, with 2 in the third line. If the leader stuck with about 200 yards for his legionary frontages, then each 480-man cohort would have covered either 50 or 67 yards, giving a depth (at full strength) of 9-7men, which would provide us with a depth of 8 men and hence a frontage of 60 yards per 480-man cohort. A 3-cohort frontage would thus give a legionary frontage of 3×60 = 180 yards; a 4-cohort frontage would give 4×60 = 240 yards.
Assuming that Roman armies in 80-73 BC operated more or less similarly to those in 60-45 BC, we can provisionally assume that
Sertorius’ opponents would have deployed in three lines much like Caesar. The new one-size-fits-all cohort replaces the traditional hastati, principes, triarii, and velites, but an enigmatic group resembling light-ish infantry, and called ‘ferentarii’ or, on occasion, ‘antesignani’, appears. These are most often employed on semi-detached service (e.g. seizing an advanced position or supporting cavalry in a skirmish), so do not seem to need separate representation and treatment for a battle.
I went with deploying cohorts side-by-side without gaps, and three deep. This will give the army a 9- or 12-cohort, 9-12 hex front line for the three legions (note that GBoH hexes mean that depths on the mapsheet are really exaggerated: in real life all three lines would fit within one hex’s depth).
Sertorius should be able to deploy more or less as we wish wish, after the Romans have deployed. His techniques seem to have been much more flexible, and he should be able to cover the Roman frontage with skirmishers backed by a single or double row of units. So for Sertorius, whilst not traditional, it is by no means an innovative arrangement.
The plan is for the Sertorian legion to hold ground whilst the Cavalry and Light Infantry skirmish with the weaker Recruit level cohorts. In the mean time, if Sullan forces expose a weakness Sertorius can advance in strength. Failing that, Sertorius will head left with the Legion and attempt to flank attack the Conscript rated (more experienced) Legion they are facing.
It would also be possible to adjust the forces of Sertorius by swapping flanks, i.e. placing the stronger Sertorian Legion on the right and having the Light Infantry play a skirmish,hit and run style delay action until the more experienced Legion could eat the Sullan forces in a combined Light Cavalry and Lancers Cohort attack. Maybe two plays are in order?!