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.
Yukon was intentionally designed to be harder for Engineers:
This abandoned mountaintop coalmine was repurposed by shadowy operatives into a top secret surveillance outpost—one which also, so nobody gets wise, insidiously continues to mine coal. There is also a log hanging in the center of the map—possibly as an example to other logs. Logs that said too much.
Five control points promise a lot of momentum swings in this compact outdoor map, with plenty of tunnels and rocky outcroppings. Plenty of alternate routes ensure that if you can’t get past a well-placed engineer through one tunnel, you might be able to sneak up behind them through another.
CP_Yukon is a community map from the brilliant dual-powered brain trust of Patrick “MangyCarface” Mulholland and Joachim “Acegikmo” Holmér.
-- Classless Update, Day 3
Point 1
Point 1 is extremely poor for defending as Engineer.
If you're building full size sentries, it may be better to build them down in the pit, although that means a Spy can back-cap the point.
Point 2
Point 2 has a few defense spots. One of the best for actually covering the point is at your team's end of the hall when you first come out the door from point 1. You had better have the Wrangler, though, or a sniper at the far end of the can easily take you out.
Another point is past point 2, on the middle route. You can place a sentry on the walkway up. The problem is that, if you place is close enough to the point to be able to see it, it's far enough from the point 3 area for Demomen to be able to take it out.
Point 3
Thanks to point 3 being moved, your best bet is near the point itself, preferably where the fence covers it, so it can't be shot by Snipers (or at least where you can't be shot by Snipers). Your biggest problem here is people sneaking up behind you. You could place it at the very end of the point 3 bridge, facing the point to help counteract that.
Point 4
Building slightly to the enemy's side of point 4 works pretty well, as it covers both entrances to the point 4 area.
Another Alternative
Due to the difficultly in guarding points with a level 3 gun on this map, Mini-sentries work fairly well on it, at least when combined with the Frontier Justice.
Best Answer
I haven't tried it myself, mind -- but I've seen more than one engineer start building a dispenser, hopping on top of it while it's still propping itself up, then using the extra boost to deposit a teleporter exit on a ledge they couldn't otherwise get to. Maybe that's what's happening?