Lumen:
I'm stumped, so Stevem please expand on your remarks - 'The bottom line isn't ?wxyz, 20D may help' and '20D is particulalry helpful, and if rearranging some of the others gives a word, then that will help too'
20D will help with the last two lines of the square, but if it were ?WXYZ this would mean ?M??P and hence N and O would both not be part of the phrase which is unlikely.
The phrase is not something guessable as it really only makes sense when decoding the puzzle's title.