@Vencini I just completed that mission. It took about 4 spotting rounds to get close to the buildings, including 1 only 25m from my observer team, and 1 that took out a squad member... ;(
As @IanL suggests, splitting teams is thoroughly recommended. For this mission I split by Assault, so each squad split into a maneuvre team (with grenades) and fire team (with MG).
Without demo or breaching charges [or a tank!] (which are what you need to hole walls) , I used my fire teams to heavily suppress the buildings for at least a minute, whilst moving the maneuvre teams as close as possible. Then time their assault into the building to coincide with the end of your suppression fire.
Likely still to be bloody, but thats urban warfare.. I think it's a great little training mission for just that, using raw organic squad firepower..
Good luck!