You can get more points out of every level if you simply do a loop. What you need to do is ignore the last piece of puzzle and simply walk by. You will start at the beginning of the level and enemies will respawn. You can merge same-level monsters to get more advanced ones, so keep collecting. The special attacks are your best weapons against superior enemies. As soon as they charge use them and you might just even out the odds. Under the coin counter you can see a video icon that will let you watch an ad to get more coins. The cooldown period between videos you can watch is five minutes. Don't use the healing ability right away. Sometimes it's better to save it and just merge the weakened critter with another one to get a more powerful critter.