I know this is going to be difficult... but you're going to need a team to do this.
Chances are you'll need at least:
- 1+ uber Medic.
- 1+ stickylauncher DemoMen as the Medics' patient(s). i.e. one that isn't a DemoKnight or Stickyjumper
- A non-YER, non-DR Spy (this is optional, but may make it easier). Red-Tape Recorder is recommended.
- Other damage classes (Soldier, DemoMan, Heavy)
- A Bonk Scout can also help to distract the sentries.
Now, the trick here is to use the Uber and get the sentries firing on the DemoMan. Meanwhile, the Spy (if you have one) should be creating a distraction by some of the farther away sentries. If possible, the Spy should concentrate on the sentry guns whose Engineers are using Wranglers.
The ubered player should be a DemoMan thanks to the Rescue Ranger... Demo is the only class whose damage can be done in burst so that the shoot to heal effect is useless (although if they pick up the gun to move it, two stickies will kill the Engineer)
The Spy's job (if you have one) is two-fold:
- Sap the sentries and supporting buildings. Especially the Wrangled sentries as you can't wrangle / remotely heal sapped sentries.
- Cause a distraction for the Engies to deal with.
The other damage classes should be assisting to take out the sentries while they are distracted by the Uber. As long as the Ubered players are closer to the sentries, the sentries will fire upon them instead. The exception is for wrangled sentries (this is one of the reasons the Wrangler is a bit OP).
Now, the biggest problem with the above scenario is if one of the players is using a Short Circuit.
The Best Solution
Team Scramble. If teams are unbalanced enough that one team can build 5 sentries out in mid, a scramble is more than warranted.