Inside Menkaure's Pyramid - Outdoors Near the Two Smaller Pyramids

As soon as you climb up through the open trapdoor, 2 giant scorpions scurry out from between the pyramids ahead on the left (southeast) and attack. If you have super grenades to spare, it only takes a couple of them to kill both scorpions. If not, you can shoot and dodge using your weapon of choice (not shown).

Or use the tunnel for cover. Shoot the scorpions with pistols as they approach. Then, when they get too close, drop back into the hole.

Sometimes they'll stay close to the opening and you can continue shooting from below.

If not, try jumping up and down while firing. If the scorpions lose interest and wander off, climb up to get their attention and repeat the process.

When you've killed both scorpions, go between the two pyramids...

...and find the button in the small alcove on the left.

Press it to open the top of the other pyramid.

This also releases another giant scorpion, which approaches from the sandy clearing near the trapdoor. If you stay inside the raised alcove, it won't attack, and you can snipe at it with pistols as it moves back and forth.

[Menkaure's Pyramid Walkthrough]