Note the two columns that only have two metal boxes each, these need to be aligned with the two line blasters (within the game) so that all four are destroyed (then two columns are free of metal boxes)
Make large matches, so that you build up in game booster rockets. Use these rocket on columns that have three metal boxes.
As you work your way down the board, use the solid obstacles in the game to slide columns over, so that two columns of metal boxes become one. Then you can blast six metal boxes away in one move!
Save at least one balloon of each color to free the Pets at the bottom of the board.