Veteran (Hard+) (超上級 Chō jōkyū): Rated for the Daily Blitz Stages and some of the first Cat Awakening stages. These stages usually need a few True Forms.
Expert (Hard++) (激ムズ Geki muzu): Rated for the second levels of the Catfruit Stages, Siege of Hippoe, some of the first Awakening Stages, and the first four Cyclone Stages. Not recommended for new players because they are pretty hard, even with True Forms.
Insane (Hard+++) (超激ムズ Chō geki muzu): Rated for the third levels of the Catfruit Stages, Sweet XP (Insane), all Crazed Cat Stages, the second Cat Awakening Stages, XP Megablitz, and the fifth, sixth, seventh and eighth Cyclone Stages. These are very difficult stages, beware when taking them on.
Calamity: This difficulty, which was introduced for the Li'l Awakens! stages, denotes levels where both 4 star and No Continues restrictions apply. So far only the Li'l Cat Awakening stages have this difficulty. This difficulty is currently unofficial and acts more as a restriction, as the Lil Cat Awakening stages are all currently rated in-game as Deadly.
For Easy levels, players at any point in the game can beat this type of level, exept at the very beginning.
For Normal levels, getting about 167% Treasures (up to the second Legendary Cat Shield) is enough to beat these stages.
For most Hard levels, getting 267% treasures is the minimal requirement for these stages.
For Hard and Veteran stages, 300% Treasures and Normal Cats at level 20 are guaranteed victories if there is also a decent strategy involved.
For Expert stages, the easiest stages are the first three Cyclone stages, then the first awakening stage of any Special/Monthly Cats. However, because the Awakening stages were released later, the player is recommended to get a few Crazed Cats or some good super rares/ubers for these levels.
For Insane stages, the player should tackle the Crazed Cats' stages first. True Forms of Normal Cats are required (with some exceptions) and effective True Forms help massively.
For Deadly stages, True Forms, sometimes, Gacha cats and very high leveled-Cats are required (with some exceptions such as Facing Danger). Manic Cats' stages should be the first group of Deadly stages to be completed. Good strategy and levelled cats are key to victory.
For Merciless stages, high level cats and well-thought strategies are always required, True Forms are almost requred, including those of the Legends; for this reason, challenging Growing Strange to get said True Forms is highly recommended.
For Calamity stages, it's important to have the help of the legends and get the rare and special cats to high levels; True Forms are mandatory and good strategies and combos are also a must.
The difficulty of levels can be very different if the player has certain units. Steel Visage is a good example of this. It is very difficult without any Critical Hit Cats, but when using units like Catornado and Waitress Cat, this stage becomes much easier.