FIRST CHALLENGE TOMB (Shrine of Osiris)
CHALLENGE TOMB ENTRANCE: After completing the Tomb of the Timekeeper, you return to the Overworld and use a time bomb to open the path to the next area. The entrance to the first Challenge Tomb is located on the walkway between the Shrine of Osiris and the Khepri Amphitheater. (screenshot) It's marked with a star on the Overworld map. (screenshot)
Inside the tomb is a water-filled channel flanked by two platforms paved with skull tiles. These contain hidden spikes that pop up when you move over them. The goal is to step on each of the plain tiles, which are safe, in order to open the shimmering gate ahead. In the single-player game, you can run the course in in one go, taking care not to run into the spikes you've already triggered. (screenshot) Or, take it one tile at a time. When you've pressed both safe tiles on one side, wait for the spikes to retract and then return to the start. Now do the same thing on the other side and the gate will open. (screenshot)
In 2-player co-op there are only 2 safe tiles. Each player should take one side and run quickly and carefully to the safe tile, avoiding the spikes they have already triggered. (screenshot)
In 4-player co-op, there are 4 safe tiles. Either plan ahead to follow paths that don't overlap or have one character on each side go first. Then wait for the spikes to retract so the second character on each side can run to the other safe tile. (screenshot)
Swim through the open gate into the next room and climb out of the water on the right. Here are more skull tiles with spikes. Some of these have gems on them. Again, pressing all 4 safe tiles opens the gate at the end of the channel. It's possible to carefully retrieve the gems and depress the 4 safe tiles, but it's easier to do the puzzle first and then go after the gems.
In single-player, run onto the first safe tile in the bottom quadrant. You can pause here and plan your next move; there's no hurry. Then take a running jump across the water and run onto the second safe tile, in the left quadrant. Pause to catch your breath then run to the third safe tile in the upper quadrant, taking care to run around the permanent sets of spikes in your path. Finally, take a running jump across the water and run around the permanent set of spikes to reach the fourth safe tile. (screenshot) You could also do it in the reverse order (counter-clockwise) if you prefer. (screenshot)
In 2-player co-op there are only 2 safe tiles. Each player can take one side and run quickly and carefully to the safe tile, avoiding the permanent spikes. This means one player will have a longer route to her tile. (screenshot) Or, you can take turns: Have one player go for the tile in the far corner by passing over the other tile. Wait for the spikes to retract. Then the second player can go for the nearer tile. (screenshot)
In 4-player co-op there are 4 safe tiles. Each character needs to stand on one of them, but rather than risking running across each other's paths, take turns. First have two characters go, so one ends up on one of the tiles on the left side of the room, while the other aims for one of the tiles on the right. Then wait for the spikes to retract before the next two characters maneuver onto the remaining safe tiles. (screenshot)
When all the safe tiles have been pressed, the gate opens. Enter and pick up the BOLT ACTION RIFLE and a MAX HEALTH UPGRADE. (screenshot) Then swim back to the exit and move on.
†UPDATE HISTORY: 12/28/14 - First draft of walkthrough posted.
ACKNOWLEDGEMENTS: Thanks to TheJollyMage for the 2-player solution. So far I've only played single- and 4-player versions.
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