This page is about the dungeons and boss levels that I highly recommend you go through. Here they are:
Squelch: 4
Evil Hideout: 12
Crimson/Frost keeper: 12
Sleeky Serpent: 13
Volcano: 20
Pheonix: 25
Ocean: 27
Afairyca: 30
Dark Ruins: 35
True Squelch: 37
Gaia Dragon: 38
Southern Dark Ocean: 40
Water Temple: 43
Aqua Tortoise: 45
Northern Dark Ocean: 50
Around Locksmiths house without Stealthy: 83
Jewel Mine: 50
Ice Pheonix: 55
Wind Tower: 60
Tempestor: 65
Shock Temple: 70
Thunder Keeper: 75
Serpent Neo: 80
Aflama/Malignus: 85
Squelch's home: 100
Master Squelch: 105
Guardian Tower: 125
Guardians: 130
Best Malignus: 155
Havoc Dragon: 175