The trick to completing this level is how you use the hammers you build up within the game, they are ideal for breaking the diagonal pattern that the colors of blocks fall into.
Firstly, pop a balloon (right side is best as it is higher in the game once the blocks drop) unlock the key, then click away the large groups you get from the blocks that made up the 500.
Save the other balloon, it is needed to free Pets from carriers in the bottom row.
Now, use the hammer either in the center, or on an outer column, on the lowest row you have, to break the diagonal pattern and create groups.
Work your way down, only using the hammers when necessary.
The two bombs either side of the game are needed to destroy all 4 metal boxes, so do not blow the bombs until there are no blocks between the metal boxes and them.
Release the Pets at the end by using the final balloon, then bring them all down.