Just to let everyone know how it went:
At the last minute, the shaman's player made it after all. So we had 7 characters. And the barbarian player forgot his character, and ended up playing a bard pregen that I had handy. So we had 2 defenders, 2 leaders, 2 strikers, and 1 controller.
The ambush didn''t get into TPK threat territory, but it was a very tough fight. With what had happened the previous session, they were already down surges. And they didn't hit their resources as heavy as they probably should. But several characters were down into single digit hit points, and all spent some time below blooded. The sorcerer hit once in 7 tries.
The party then preceded to turn it up a notch, and waltz through the next encounter. By now, more than half the dailies were gone, and some were at zero surges. They got a little cautious at that point, but I gave it to them straight. I thought they had enough for one more tough fight, but that was it. They had enough information to think that one more tough fight might do it. So they turned it up a notch again. We ran through an ad hoc skill challenge that let them do a bit of divide and conquer, turning an encounter level +3 boss fight into three close but separate waves (no short rests in between). The shaman had to leave.
Then I started rolling under 5 for the foes abilities to resist bluffs, spot hidden characters, initiative, everything but hits. At one point, they had set up their own ambush such that when the rolls finally broke their way, man did it break. They all go a surprise round, perfectly placed. Even the warden with her 3 Stealth skill rolled a natural 20 and surprised the foes. And then they all won initiative. They thought about using action points, but by that time, we were clearly in the mopping up phase. To add insult to injury, the surviving foes all missed their first attacks.
So in the boss encouter, the party got three full rounds of essentially unanswered attacks, and seldom missed.
The only downside was that the wizard was a little put out that her plan to push monsters into a pit never came to pass. By the time they got to where she could push them, everything she hit them with killed them. So she did pile up some bodies in the pit.
