Look at Captain Keyes’ abandoned pipe
Halo Combat Evolved MEGG – Achievement Guide
Complete Skull and Terminal CHECKLIST(Halo 1-4 / Google Docs)
After you started the first mission on legendary, turn around and crouch jump on top of the containers at the end of the room. Wait for about 20 SECONDS and continue with the story until you get your weapon from Keyes. Collect the ammo and kill the 3 grunts. Return to Keyes and shot him in the head.
A couple of soldiers will now come running for you. You can’t kill them. You have to hide and wait for the right moment to run past them and get to the newly opened door to the right of Keyes room. This can take a few tries but you can use your CHECKPOINT and still get the achievement.
Once you are past them, you can find blood on the ceiling in the back of the next room. Look at it and the achievement will pop.