On level 80 you need to concentrate on making matches in the top part of the field. For that you need to get rid of the crates on the sides. Destroying one is enough, but it would be great if you manage to get both of them. You can destroy them by making matches in the bottom part of the field, and once you are done with that, you can forget about that area altogether (as long as there are moves in the top part, of course).
It might seem like destroying all the crates would work great, however, it is almost impossible to achieve this with the number of moves available. On the other hand, if you are successful at quickly destroying the crates in the corners and making the best matches possible, the level will get much easier!