What was the Last Stand again?
A multiplayer mode?
EDIT: Oh, a 3-player survival mode...nice. Can this be played solo with 2 A.I. "friends" ?
You cannot play The Last Stand with A.I., unfortunately. Well, perhaps I should say fortunately instead. It requires good coordination to have a successful match (all 20 waves completed). I don't think that any A.I. would do that, they'd just rambo on the closest targets available. That's not even mentioning the different builds for each heroes and how game-play styles differ according to the weaponry and accessories you equip. With this said, however, some accessories for your hero grant you an A.I. unit (or more than one in some cases) to your side to aid you (and the team in general), for support and/or enemy distraction.
In my opinion, The Last Stand is the best part of Dawn of War 2 (typical skirmishes are alright though). I play it almost daily, at least two or three matches each session. The only hero I do not own is the Tau commander since I never bothered buying it (he looks fun though), but I have all others. I like all of them for different reasons, but my favorites are the Chaos Sorcerer, the Imperial Lord General and the Eldar Farseer. Now, with this said, there is a negative point to the Last Stand mode: there's only two maps. Namely, the Bloodied Colosseum, and The Anvil of Khorne. My personal favorite is definitely The Anvil of Khorne. The 'special' thing about Colosseum, however, is Wave #15, it's a mirror match (all three heroes are 'cloned' and you fight yourselves; the clones have the exact same build as you and your two allies as well).
I simply adore The Last Stand, and I HOPE that they'll bring it back for Dawn of War 3... with more than two maps. But I also hope that they keep the 3 heroes formula and don't turn in into a 5+ heroes mode with maps that would be too big and unbalanced. The one thing I wish they'd include would be Waves customization. If they do that, to any extent, I'll be in heaven.