This looks like a first. The AI never reacts with the Romans, and loses to Frankish bow fire alone .
The last unit to break was Julian and his cavalry bodyguard.
Although its an exercise in tedium, you can rout the Romans with the LI archers only. The Roman LC responds meekly one at a time. Two cataphracts eventually moved, one of which died quickly from missiles, another routed a LI javelin unit but never moved again. After that the Romans never moved anything and were destroyed one unit at a time.
The Roman AI is set to avoid attacking the Germans until they come off their hill. As you say, this does not take into account tactics such as you suggest. We will look at tweaking it - but we can't make it too easy to lure the Romans up the hill.