Sambro - You can immediately shade in the middle square on the top row. This is because 3d must be mirrored by 4d if you fold the puzzle in half. You have then established the starting point for 4a which must then be 7 squares long (or less).
Now write 24 on the bottom row opposite 4 and blank out the square on its left. Remember the puzzle's symmetrical form by mentally folding it in half both horizontally and vertically. Have you shaded the square to the left of 15a and its opposite? Also shade above 8d plus three more 'opposites'. Try setting up the whole grid without answering any clues, which is a puzzle in itself, though not necessarily easy.
Hope this is of some help. ☺ja