Did Sleeper really have D12 physical stress already? I thought he was quite a bit lower than that.
If you do wish to avoid the damage, as an alternative to spending a plot point to add a D6 or D8 to your dice pool (which I think is normally supposed to happen before you roll the dice), you can spend a plot point to add a third die to your total, and you can do this after you see the result. So in this case, you could spend a point to add the 9, 7, and 6 together, and that would be more than enough to beat Electro's 17.
Another option would be to spend a plot point to turn the D8 physical stress you are taking into D8 mental or emotional stress instead. I don't know why you would do that instead of adding an extra die to your total and avoiding it all together, but it is a way that you can spend plot points.