Two sources use different difficulty words, and they should stay separate until the game shows they are the same button.
What the game page says
Trout Land's description says Casual to learn and Expert to suffer. It does not mention Regular or Simple, and it does not define a contract.
What one entry screen says
On the walkthrough chapter Tin Can Tex, about 17:43, the entry screen shows three rows:
| Row | Text on that screen |
|---|---|
| Casual | Free retries. Earns the contract on Regular! |
| Regular | The real fight, earns the contract in full |
| Expert | Locked, with Beat it on Regular to unlock |
On that encounter, a Casual clear is described as practice that points you at Regular for the contract. Expert is not available until Regular is beaten. This is one encounter's panel. Check the next stage instead of assuming the sentences are global.
What results cards say
No-hit clips of the clock boss, the tea room, Wobbles, the cloud, the barnyard, and the balloon boss all print SKILL LEVEL SIMPLE on the results card. They also show an HP bonus based on 5 health when no hits land. The word Simple does not appear in the official description. It may be the in-fight name of the learning difficulty, or a separate setting from Casual. The checked material does not decide.
What to do with that
- If you want the contract described on the Tin Can Tex screen, clear Regular on that encounter. Casual's own text says the contract is earned on Regular.
- If you are copying a no-hit video, match Simple, because that is the skill level printed on those cards. A Regular or Expert attempt is not shown in those clips.
- Do not treat a full HP bar during the fight as the same thing as the end card. The card is the confirmation.
There is no published damage multiplier, parry window, or retry difference beyond the sentences above.