Pet Rescue Saga Level 521 Tips
Start clearing the blocks of the level and try to keep high the locked blocks, clearing blocks aside them. Use the hammer booster (that gets activated) to destroy the block between two pet boxes, in order to rescue the pets there. When all these are done, and you have no other things to do, use a key to destroy the locked blocks and repeat the same procedure again, in the part of the screen there have been the pets.
Pet Rescue Saga Level 521 Walkthrough Video
The following walkthrough video will give you a few more ideas on how to complete level 521!