Legolas (attached with the Blade of Gondolin) used Rain of Arrows (pre-combat stage) to eliminate a Hill Troll's final HP that was engaged with the other player. Does Legolas get to use his and/or the Blade's Response special ability of adding progress tokens to the quest (or buffering active Enchanted Stream location in this instance)?

I know I read this somewhere before, and maybe it's my turn to be overlooking the answer in the FAQ, but I believe the answer is "no, neither Legolas nor Blade of Gondolin's Response effect would trigger when using Rain of Arrows." If I recall correctly, the logic is that you exhausted Legolas to play an event that has its own specified damage (and makes no reference to the attack values of heroes that trigger it), and not to initiate a direct attack against an enemy.

Maybe the answer is somewhere in this thread (I don't have time to check)? http://www.cardgamedb.com/forums/index.php?/topic/1702-official-nate-rule-clarifications/

If you find the official or quasi-official answer somewhere, please share!

TwiceBornh is correct and the answer is no, Rain of Arrows is a direct damage card effect, and there was no "attack" made (an "attack" being: exhaust a character as an attacker, compare attack str vs. defense str of the enemy, assign damage...). Legolas and the Blade's effects only trigger after he participates in an "attack" that destroys an enemy, per their card text.

Cheers! Thank you very much for the clarification!