Pet Rescue Saga Level 521 Tips
Start by clearing the blocks by the locked blocks and try to keep them (the locked blocks) high! Then use the hammer booster to destroy the block between two pet boxes, in order to rescue the pets. When all these are done and nothing else can be done use a key to destroy the locked blocks and repeat the same tactic again, in the part of the screen where the pet are.
Pet Rescue Saga Level 521 Walkthrough Video
The following walkthrough video will give you a few more ideas on how to complete level 521!