This doesn't necessarily require a video, so for those that would like a short and sweet text blurb, here you go:
In the Crystal Caves, you'll need to enter the Dragon's Den in order to get the rune to finish the level. The final stretch of the level with the Rhinotaurs will have a cave under a small waterfall. The water itself is shallow, meaning Dan can enter it (this is where it tricked me at first). Inside, you'll find a small trove of gold, the rune, a few energy vials, and a Dragon statue. Without the Dragon Gems, this is simply a trove.
Each of the four main platforms have colored crystals on them corresponding to the crystals hanging above. Break out your hammer and slam down on the ground to see that it will force a crystal to fall down. Wallop one, jump onto the platform, and be hit by the crystal to get the trophy. It does not require you to be fighting the dragon in order to get the trophy.