Killing the final cloaked Elites
As you return to the top floor of the main facility, you've got multiple cloaked Elites to deal with. But they're easier to handle than you might think. Position yourself just far enough forward to be able to see the head of an Elite in the first little chamber when one stands there, then administer pistol shots to the head. Wait for the next Elite to come see what all the fuss is about, and repeat. Of course, you can also lob grenades in to help things along; but preferably wait until a few Elites have dropped, so you're likely to get a huge chain reaction. For more visceral fun, use a rocket launcher that you left around deliberately.