You need to hijack an Atlas mech by cracking its windshield, then killing the driver, and then walking over to the mech and entering it.
While this seems easy, I struggled on Normal difficulty, because I kept destroying the mech instead of killing only the driver. On Hardcore I got in on my first try. I suggest players on lower difficulty levels to turn it up temporarily for this trophy (and tell your squad to hide somewhere and not use powers).
Notable opportunities occur at the end of Eden Prime, where you can climb on a building and take out an Atlas from above, and Grissom Academy, where you can engage in a bit of mech-vs-mech fighting. This minimises the danger of being killed yourself by the Atlas.