So I had Visionary against Voss. Visionary had previously played a Decoy Projection that had 2 HP left when a Forced Deployment came up. She proceeded to Brain Burn the villain trash (13 cards), redirecting the damage to the Projection. I assumed that all 13 damage went to the Projection, massively overkilling it and saving Visionary. Did I play wrong?
You did it correctly. The damage is dealt at once, not one point at a time. The decoy would be at -11 HP after Brain Burn resolved and would be destroyed.
Oops, I've been doing that wrong, then - I should know better, really, after all this time, lol. Say my Decoy has 3hp left and gets hit for five points of damage, I'll take 3hp off the Decoy which kills it, so then Visionary takes the final 2hp. So basically I've been completely forgetting the "stuff can go into negative hp" thing that I remember for everything else in the game, lol. Not that I play Visionary that often, and when I do I rarely get a Decoy out anyway, so it's not something that's ever really come up.
It goes by instances of damage, not points. This matters for those effects that hit everything twice, such as Char, Explosives Wagon, Cursed Acolyte, and "Diplomatic Envoy". In all cases, if you have a Decoy Projection or Stealth Bot out and one of these effects hits all hero targets, you can first choose to resolve the damage against one of the heroes. You redirect the first instance to Decoybot, and if they're still alive, you then redirect the second instance. You then move on to the next hero. If enough damage reduction is in play, you might manage to have the Decoybot still be around after each hero character card has been damaged - in which case Decoybot itself is also directly targeted by the blast because it's a hero target.
For example, Char is in play and there are no bonuses to damage; Stealth Bot has 5 HP and it innately reduces all damage it takes by 1. Char hits Unity for 2 fire and 2 toxic, which becomes 1 and 1 to Stealthy. Then another hero takes 2 and 2, which becomes 1 and 1 to the bot. Then a third hero takes 2 fire, which becomes 1 fire to Stealth Bot, killing it, and then that hero must take the 2 toxic.
Example 2: All other heroes besides Unity are incapped. Char hits Unity for 2 and 2, which becomes 1 and 1 to Stealth Bot. Char then hits Stealth Bot for 2 and 2 which becomes 1 and 1. Assuming Stealth Bot was at full, she survives, but if even one hero was alive and redirected damage to her, she wouldn't.