Square of the Stream
The square of the Stream is the home to the Element of Water. There are a total of three tasks in this map, handled all within one visit.
When you enter, there are up to three werejaguars in the courtyard ahead. There are three exits, but the one you need is to the right - the others require a key item or are barred. The right-hand exit leads to an underwater tunnel.
The underwater tunnel shouldn't post too much difficulty if you previously got a ring of water breathing. If not, you simply have to head to the air holes along the route, and defeat the hydras quickly. The route through the tunnel starts with a left turn, and basically alternates between the extreme left and right sides of the maze. At the end you can surface, and collect the element of water in the next room (although there are four guards.)
Shortly after collecting the element of water, the wall facing the stairs will open, providing a quick return to the yard.
The left exit leads to an open area, with a mystic urn on an obelisk. Grabbing it is as easy as getting on the nearby platform, and jumping towards it.
For the Serpent's heart, enter the passage below the way you came in, and turn left. There are two werejaguars in the room. Heading to the far side of this room and stepping on the floor switch will raise a center platform which has the serpent's heart.
When returning, there's a side passage that you can use as a shortcut.
Take the final exit from the center. There are a few enemies here, but are not a threat.
When you place the heart on the altar in the middle of the room, a platform will rise out of the water, holding the crystal skull. Take it.
A skull wizard will teleport in after you retrieve the skull, and two more will appear in the main yard. You can also return to the hub at any time.