What is a good strategy to deal with lots of engineers turtling on the other team?

Solution 1:

Lots of medics with lots of ubers on high-damage-dealing classes. Demomen, if they are any good, usually work best against a sentry farm — stickies are perfect for this job, the usual problem is living long enough to deploy and detonate enough of them.

Heavies or pyros can also be effective, if they can get close enough (short range for a heavy, point-blank for a pyro) without being overwhelmed by the knockback before they can do damage. The medic should lead the uber combo in (particularly if the partner is a heavy) — he's faster, so he can absorb the knockback better.

If at all possible, back the uber team(s) up with as much raw firepower as you can muster — the aim is to either overwhelm the engies' ability to repair stuff, take out the engies directly, or take out dispensers. Soldiers are good for this support role, as they can place well-aimed damage from behind the uber. Demos can support well, too — their indirect fire can damage engies and dispensers without having to kill the sentry first.

Sending a spy in to sap just before or during the uber(s) can be a good idea, too.

Solution 2:

Unfortunately, twiddling your thumbs behind your own base isn't going to win your team the round. If there are that many Engies, Spies will probably be ineffective.

Grab yourself some medics and frontline (or take 'em from behind) forces like Heavies, Soldiers, or Demomen, uber, and advance. Side tip : as you advance, have your progress reinforced by your own engineers setting up on newly-won terrain behind you. This will help your new ground to hold out longer.

Class updates will always create all-of-a-sudden influlxes of the respective classes, but just remember their strengths and weaknesses, and don't feel too overwhelmed.

Solution 3:

A lot of engineers usually involves a lot of teleports.

As a spy, alert your team about your intentions and go to sap the teleports entrances. These will cause the engineers move the focus out of the sentries and your team should be able to put them down. 1 uber or 2 could help. ;)