How could you possibly clear the 4 armored guys on glass without using the gun?
You can't kick, because you can't kick when you're seen. You also can't go invisible for the same reason.
And you can't grab them because they're armored.
If there's any way to do it at all, I expect it to be exploit, but as of right now, I don't see any way to do it.
You were right about every other level, though. Thanks for the challenge.
Edit: One bullet on that level may be possible, but extremely hard.