Playing as Carthage i use 5 onagers with 10 sacred band infantry,3 war elephants and 2 cav(general inc).I take out the gate,gatehouse,the 2 walls immediately to either side of the gatehouse plus the 2 towers at the end of both those wall sections.I put the SBI on slow approach to the gaps in the walls and the gate and just before they reach these gaps i charge the elephants through.The SBI then have no problems with being flanked by the shattered units on the other side of the walls.
As the Julii i do something similar except with Urban cohorts who do not need the protection that the SBI require so no elephants needed there.