I assume you are laying hot seat or against the AI ?
I only ask as the difference between a human opponent and the AI in this type of scenario is huge, the tactics that the Germans need to use depends on which strategy the defender uses, as Ed says the Belgian player can win by either evacuating the city an moving the 3rd Div to the exit hexes or to stay and defend the VP hexes within the city, as Ed says there are only vp points awarded for the forts to prevent the Belgian player from not garrisoning them correctly, if you look at the VP levels you would see that the German player does not need to capture any fort to win the game and so they can be ignored by the German player.
So as Ed as us you need to surround the city to prevent the 3rd Div from reaching the exit hexes should the defender try that tactic, if he defends the city you still need to surround the city as that will drop the defenders supply from 70 to 20 and cause more units to go low ammo, aim your attacks at the gaps between the forts especially to the north of the city as there is at least three hex gaps between the forts, attacking from the south across the river will not succeed against a competent defender but you need to threaten all of the perimeter to stretch the defenders as thin as possible.
If the Belgian player does decide to defend the city then it is a race against time to get to the VP hexes so some chances need to be taken with stacking, the Belgian player does not have any arty so some risk with stacking is acceptable in this scenario.
I have played this scenario as both sides and won as both sides, so both results are possible.