This puzzle is a sudoku, using letters instead of numbers. The first step is to place the answers to the eight feeder puzzles into the highlighted spaces in the grid. We are told that all words go top-to-bottom and/or left-to-right, so HEN, FIGHT, and THE BFG can all be placed due to their unique lengths. The two four-letter answers which intersect must be FEST and SIFT in some order. The bottom row must contain BITS or FISH, since the E of FINE cannot be in the same column as the E of THE BFG.
Since both BITS and FISH place an S in the bottom center region, SIFT cannot be the word in the eighth row. It must be in column nine, with FEST intersecting it. The S of FEST forces FINE to be the word in column eight, with FISH in the top row, and BITS on the bottom.
Once the words are placed, it is a simple matter to complete the sudoku. The finished grid looks like this.

The answer is found on the diagonal from top-left to bottom-right – BIG FINISH.