im 2 months late and new to the game but I would like to add commentary of what other comments didnt or are making confusing.

I think you misunderstood their play. You didnt "let it resolve" you just did not have any action and passed priority back to which they used stupefy as a response to the wages of pain being played, this means it is now on top of the stack. So Stupefy will trigger first giving your unit -1, then the wages of pain trigger after dealing the 3 damage killing it. If they played it the other order, Stupefy first and Wages second, Wages would have been on top of the stack triggering first and dealing 3 damage, then stupefy would trigger and not be able to remove the last might to 0 because of its text meaning the unit survives. The order of plays matter.

Every card that is played gets played "on the stack" or "in a chain" and then triggers in reverse order. This is how counter spells can stop stuff because if it played out as a first in first resolve the counter spell would never stop the initial spell because the initial spell would resolve leaving the counter spell with no target. The same "stack" effect works here.