All Legend Stages are rated by difficulty. They can be viewed in the sub-chapter's/event's information. (needs picture)
Easy (初級 Shokyū): Rated for the first and second levels of many events, same difficulty as some Empire of Cats Chapter 1 or 2 levels.
Normal (中級 Chūkyū): A little bit harder, most of them require more than 100% Treasures collected from Empire of Cats to beat. Rated for Catburger's Awakening Stage, among others.
Hard (上級 Jōkyū): Moderately hard most of the time. Rated for Cat Ticket Chance!, every last level of the weekday stages (excluding Weekend Stage) and for the first levels of the Catfruit Stages.
Veteran (Hard+) (超上級 Chō jōkyū): Rated for the Daily Blitz Stages and some of the first Awakening stages. These stages usually need Bahamut Cat.
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, 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: Currently the highest difficulty (up to 9.1). Rated for Uncanny Legends, Merciless XP!, Growing Strange, the tenth Cyclone Stage, all of the later Advent Stages and some collaboration-exclusive stages. Easily the hardest stages in the game, you must be prepared to tackle these.
Calamity: This difficulty, which is exclusive to the Li'l Cats Awaken! Stages, denotes levels where both 4-star and No Continues restrictions apply. This difficulty is currently unofficial and acts more as a restriction, as the Li'l Cat Awakening stages are all currently rated in-game as Deadly.
Recommendations
For Easy stages, players at any point in the game can beat this type of level, exept 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, sometimes, Gacha cats and high-level are required (with some exceptions such as Facing Danger). Maniac 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 generally required, 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.
Notes
The difficulty of levels can be very different 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, this stage becomes much easier.
