About all I can say in defense of this scenario is that it is entertaining.
We played it 3 times today. I had some thoughts on how the Romans could win it but my opponent is our traditional Roman player so he was the Romans first.
First game: Romans started with a Line Command but got unlucky and only scored 2 hits while the Gauls ran like the wind. Then his cards went bad on him and I easily won, 6-1.
Second game: My opponent wanted to get the taste of that drubbing out of his mouth so he played as the Romans again. He started with a Move Lights and it was more engaging but the net result was still the same, 6-3.
Third game: I got the chance to be the Romans and got good cards. I started with a Line Command and did some decent damage, retreats against him opened the distance between us. He did an Order Medium and moved his Warriors and some Cav out of the way and opened retreat paths for his units, which is what I didn't want him to do.
I did an Order 3 Units Right and pushed my Heavy and 2 Medium Cav up and got unlucky, only killing 1 unit and almost, but not quite killing the Warrior with the Leader, which I REALLY needed to kill! He countered with an Order 4 Left and killed my 2 Mediums with his Warrior. My Cav leader ran through his units to an Aux.
I did an Order 3 Units Right again and my Heavy with a Leader rocked, killing 2 units while my Aux did a number on his remaining units. Score was now 3-3 in my favor. He retreated 2 units off the map and I saw the handwriting on the wall.
I did a Coordinated Attack and my Medium on my left flank killed his Light Bowman on his far right flank that had been ignored and my Heavy on my Right killed the last unit on his left, bringing the score to 5-5. I used a Light Bowman to shoot at an Alemanni Light that had 2 hits left but both dice missed. He retreated a unit off the map to win the game.
Analysis: I had a good plan from the beginning and had good cards. With the exception of not killing the Warrior with a Leader, I had good luck and still lost. There is no way this scenario is predictably winnable by the Romans.