I guess that's another thing like beating Master Stalfos that LTTP teaches you.
(I think the first time I came across Master Stalfos, I used Bomb Arrows – you know, equipping the Bombs and the Arrrows and hitting both buttons at the same time. This rather badly bugs out the game, though: bomb arrows detonate immediately, so you can in fact land enough hits on him to kill him completely the first time you see him, and he won't drop the Hookshot! Oddly, I've never seen this mentioned anywhere online.)
Just another perfectly ordinary small key – except you need every key to make it through the dungeon, if I'm not mistaken, so you need that one too. It's at 9:12 in http://www.youtube.com/watch?v=rXKetQj9oQQ . (I guess it at least looks vaguely conspicuous?)Can you explain which key in Turtle Rock dungeon you're talking about?.. I'm just curious.