Nigel's response is totally correct, but just to be absolutely clear about the response trigger:
Page 23 of the manual:
Responses are denoted by a bold “Response:” trigger on a card. Responses are always optional, and can be triggered by their controller in response to (i.e. immediately after) a specified game occurrence.
Response: After Legolas participates in an attack that destroys an enemy, place 2 progress tokens on the current quest.