I've never been unable to blow the first wall on CC.
What often happens is, as a Heavy Engineer, start the match, buff myself, and then a Light/Medium who starts running to the objective. Get carried halfway effectively, means I generally get near the wall before the Resistance do. Stick a turret (only on Medium but works like a charm) pointing in the general direction of the gap in the wall, and then use my buffed Maximus to start whittling down people through the gaps in the wall. It's like a freakin' LMG sniper rifle with a red dot sight, assuming you burst fire. That gives plenty protection to any soldiers on your team planting charges, and if they're still getting through, or you have a lull in the action because of people respawning, stick a mine either under the gap (works every time because they can't stop mid-slide) or if you're getting flanked, put it at the top of the wall to the far-right that both you and the team can climb over.
This is, of course, assuming you're doing ALL the work.
Keep in mind, that the Sec can get to the wall a little before the Res, and any soldier bar maybe a heavy can get the charge up before they get cut down, and once it's there, it should be easy to defend.
Other general tips are Caltrops/ satchel charges around that gap, and dedicate a couple of soldiers/operatives to go over the wall to the right and cause some mayhem. A light engineer/operative can go over there, run across the bridge, drop down and cap the Health command post easily if their team is tied up at the first objective. And with any luck, you can upgrade/firewall if too, and suddenly it's a lot harder for anyone to stop your team.
Of course, once you manage to get locked down, all I can recommend is any heavies and mediums change a weapon to the Lobster/EZ Nade briefly, carpet bomb the points where the enemies are gathered, and let the lights (have them mostly be medics just for this) leg it to more advanced cover, heal up, and go over the wall/through the gap and do some suppressing fire.
If your team has any semblance of co-ordination, or brain cells, most of this should be fairly automatic.
Good luck, men.