What happens if this KO’s your opponent’s last character and your last character at the same time?
[Source: WotC]
In that case, the game is a draw. Time to start the next game!
I have a tapped
Flamewar - Veteran Decepticon with 3 life remaining with 2 other untapped characters and a facedown
Bolster. My opponent has 3 untapped characters and taps their attacker, declaring Flamewar as my defender. Because the conditions are met, I reveal Bolster and play
Multi-Mission Gear onto my Flamewar. That triggers Multi-Mission Gear and allows me to play an action, which I choose to play One Shall Stand, One Shall Fall targeting my Flamewar and one of my opponent's characters. This kills my Flamewar but the attacker still lives. What happens in this scenario? Does my opponent lose their attack because the defender was killed prior to battle cards being flipped? Or does my opponent get to choose another attack target?
[Source: WotC]
Your opponent has already chosen who they are attacking, so the attack would end because there is no longer a defender.