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Edited by patrickmahanDesperate Hour - Can Vader win alone?
Depends on who the heroes are. Due to Gaarkhan having a RY weapon, plus his skill to turn a focus into a Red die, while wounded he was adding 2 damage, plus the 2 damage for negating 2 surges mod item for his weapon, he did like 12 damage to my Vader on one swing (lucky dice roll for him and crappy for me).
Let's just say I was planning on letting them win, but that def made it a round or two shorter! LoL
~D
If you have Vader in Desperate Hour, him becoming defeated does not end the mission. 
Darth Vader isn't in Desperate Hour unless you add him through an open group. Did you mean Last Stand? I'm also not sure what you mean by "win alone".
The objective of the mission "Last Stand" is to defeat Darth Vader. As long as he's on the board, the Imperial can't lose. But I seem to think the question more revolves around the fact that Vader gets an action after every Rebel activation. So Vader gets 4 actions per round rather than the standard 2, and since Vader gets actions instead of an activation, he can attack 4 times per round if desired.
Since the Rebels are still under a time limit, and they may have some damage prior to opening the room with Vader, it's not impossible to think that Vader can wound all 4 Rebels by himself. Although, the Rebels should be plenty beefy by the last mission. So a focused and coordinated attack against Vader should take him down within 2 rounds as long as the Rebels don't waste effort on any other Imperial figures on the battlefield.