This is an annoying level where sometimes you do everything right, then get to the last few rows and are suddenly 'out of combinations'
Be aware that this level also asks you to save 6 Pets, but gives you 8, so it is ok to lose one or two on the way down, or to not save them all.
I approach it in sections:
First you need to concentrate on getting a key out of the glass box.
Two keys is even better, (so you don't have to do it later).
Three keys this early is ok, but could be a problem if you run out of combinations on the way down your board. The third key is needed for the very last row.
Open a key, clear all the blocks you can, then use the key to unlock the first three rows of locked blocks.
In the next section you have to get the board low enough to see three more rows of locked blocks, then use another key.
Save the rocket you build up because you might need it in one of the first two columns to lower the pets.
When you have cleared all you can, look for three rows of locked blocks.
If you cannot see the third row yet, use the rocket in your highest column.
If it still does not drop enough to see three rows of locked blocks, or you haven't been able to open the key, quit and start again.
From now until the bottom of the board you'll need to clear all the blocks as efficiently as you can.
Look out for 'odd' colored blocks without matches, plus a metal box.
You can destroy these (if they are under pets) with the rocket you build up within the game, or the line blaster (spring) you find along the way.
Once you're at the bottom row, use the key to unlock, and hopefully your blocks will co-operate and match to be cleared!