Treasure from monsters killed by reactive damage such as Fire Shield
Since this has been open for too long, I'll throw in my interpretation of the rules.

I think the first step is to be clear about the turns. It seems you would be drawing a treasure on someone else's turn.

The rules talk about a Player Turn (sometimes Hero's turn) on page 6. It even singles out how each player's turn is defined by the Hero-Exploration-Villain phases (page 4, under "Taking turns").

This is important because the other player's turn is totally independent from your turn, so we don't have to take into account your previous actions.

On to the the ToA rulebook (emphasis mine):


  When your Hero defeats a Monster, draw a Treasure card (see 
  page 13). You can only draw one Treasure card per turn, no 
  matter how many Monsters you defeat during that turn.


First, I'd rule that your hero has indeed defeated a monster. So we're clear on the first sentence.

As for the second sentence, I'd rule that you haven't yet drawn a treasure on that other player's turn.

So... use that treasure wisely! ;)