With all four elements in possession, Sam is now close to Memphis, where he hopes to find answers on how to activate the spaceship the Sirians left, but must first eliminate the enemy resistance guarding the entrance to Memphis.
Past the starting hill (the starting area), turn and walk right until you see a Large Health. Pick them up to register the secrets and spawn some enemies.
From #1, continue walking right to some oasis with palm trees and some powerups. There is a specific spot where you must stand on to register this secret, it is near the rightmost position/tree of the oasis. Hurry up to get it registered though, as Harpies will spawn and you'll receive heat damage if you stay too long there.
As of #1, turn and walk left instead, there are some powerups. Picking them up will register the secret, and like #1, spawn some enemies.
From the wall by the city entrance, follow it to the left. You'll see a tree. Blow it up or cut it with the knife to spawn enemies and items.
(HD-only) Opposite of #4, there is a +1 health pill next to a ledge attached to the wall. Pick it up to spawn some enemies.
When inside the city walls, in the area with the XL2 Lasergun, head for the left corner and rocket jump to the building/platform (can only be done on Normal or higher difficulties) or jump on top of the enemy and then on the building (on Tourist difficulty). There's a small "hallway"/corridor in it, with some ammo. Pick up the ammo to register the secret.
Opposite of #6, rocket jump onto the right building/platform (or jump on top of enemy and then on the building [on Tourist difficulty]), and then get off it immediately, but head for the center of the area, as 2 Majors will spawn near the Rubble. After the rubble vanishes, pick up the +100 health to register the secret.