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 can be pretty hard, especially without True Forms.
Insane (Hard+++) (超激ムズ Chō geki muzu): Rated for the third levels of the Catfruit Stages, all Crazed Cat Stages, most of the second Awakening Stages, XP Megablitz!, and the fifth, sixth, seventh and eighth Cyclone Stages. These are very difficult stages, beware when taking them on.
Deadly (極ムズ Goku muzu): Rated for a few levels that have a unique boss that appears only in that level, such as Advent Stages and Maniac Stages, to make few examples, as well as XP Colosseum, the ninth Cyclone Stage and all Stories of Legend stages from Stairway to Darkness onward. It is also not recommended for new players. They usually do not allow continues. Most Deadly stages can only be played after completing Into the Future Chapter 1.
Merciless (超極ムズ Chō kyoku muzu): Currently the highest difficulty (up to 9.7). Rated for Uncanny Legends, Merciless XP!, Growing Strange, the tenth Cyclone Stage, the later Advent Stages and some collaboration-exclusive stages. Easily the hardest stages in the game, you must be prepared to tackle these.
Demon (鬼級 Oni-kyū), God (神級 Kami-kyū) and Dragon (竜神級 Ryūjin-kyū): A set of difficulties exclusive to the Castle & Dragon Collaboration Event, they are roughly equivalent to Veteran, Expert and Insane, respectively.
For Easy stages, players at any point in the game can beat this type of level, except at the very beginning.
For Normal stages, getting about 167% Treasures (up to the second Legendary Cat Shield) is enough to beat these stages.
For Hard stages, getting 267% Treasures is the minimal requirement for most these stages.
For Hard and Veteran stages, 300% Treasures and Normal Cats and Bahamut Cat 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/Uber Rares for these levels.
For Insane stages, the player should tackle the Crazed Cats' stages first. True Forms of Normal Cats are often required (with some exceptions) and effective True Forms help massively.
For Deadly stages, True Forms (particularly Bahamut Cat's), Gacha Cats and high-level are recommended (with some exceptions such as Facing Danger). Maniac Stages should be the first group of Deadly stages to be completed. Good strategy and leveled cats are key to victory.
For Merciless stages, high-level cats and well-thought strategies are always required, True Forms are generally required, including those of Legends; for this reason, challenging Growing Strange to get said True Forms is highly recommended.
The difficulty of levels may vary drastically if the player has certain units. Facing Danger is a good example of this. It is very difficult without any Critical Hit Cats, but when using units like Catornado and Waitress Cat, the stage becomes much easier.