This came up as a point of debate in a game recently.
The card reads "Exhaust this card after resolving an attack using "Havok Shot". Each figure that suffered one or more damage chooses to suffer one damage or become Stunned. If that figure is already Stunned, it must choose to suffer one damage."
Now my take on this is that there are two triggers.
1. Did you resolve an attack using Havok Shot?
2. Did the figure suffer DMG?
I believe this applies to the initial target of the attack as well as all figures hit by the blast. Another player was saying that this does not apply to the initial figure and only those hit by the Havok Shot.