Go over

After you take the first point on the second stage, your going to end up going through that little tunnel. The sentry guns are going to be waiting for you, and with such the rest of the team should be watching either side of the tunnel to suppress the people smart enough to not come right out the middle.

Ive seen this strategy work a bunch of times for clearing out of the tunnel. You need a Scout and a Demoman to pull this off. Since the sides of the map are being watched and spammed by Soldiers, Demo's, and Snipers, we go right up the middle and over the sentry's. The Demoman, or 2 Demomen lay down a single sticky bomb a piece. Have a Scout run out as bait for Snipers and initially for the sentry guns. The moment he heads out, the Snipers should pick up on him along with the sentry guns. Demo's arm the regular grenade launcher and do a pipe jump straight out and right above the sentry guns. Coming in they start piping on the SG's and Engineers, hopefully killing the Engineer at the least. While this is happening, your Sniper is moving up the tunnel a bit to take down the opposing Sniper. You have a Soldier or 2 hop out from the right side ledge and spam on the SG's and Engineers and those suckers are down fast.

You can do a similar job on the next bottleneck which is right before the cap. Have the Demo's jump up on top of the building where the spawns occur. The SG will be shooting them from behind, which actually helps to force you in to position a little faster. They can then pipe the SG which is down in the cage. This opens up a lane for soldiers to spam the SG at the top of the steps.

Of course you want to keep a Heavy/Medic team and Pyro's to fight back charging enemies to make room for the Demo's and Soldiers to do their stuff.

Medics need to do a good job of buffing up the Scout for the first jump as well as the Demo's. They are crucial for that first portion, and will likely (unfortunately) be a casualty.