What's the most effective way to deal with a big group of sentries? [duplicate]

I'm playing Capture the Flag (on Turbine), and the enemy's team has a group of about five level-3 sentries right outside the door to our base. I've tried being a Soldier, Heavy and Demoman, and destroying them, but all five gun me down before I have the chance. The Scout's essentially useless and he'll be destroyed almost instantly, as will the Pyro and Medic. As a Spy, I am either blown up by grenades or sticky bombs before I even get near the sentries, or, if I'm lucky enough to get to them, as soon as I sap them, another Engineer comes along and removes my sapper (and I obvious can't backstab him as other Sentries will quickly shoot at me).

What is the best thing to do in this situation (in order to destroy lots of sentries), where coming into contact with them is inevitable (What is an ideal class to use, and what is the best strategy)?