If I remember correctly, the cheat to lower your level was added a long time ago when capturing monstergirls was intorduced, since some people were so strong, that it was difficult for them to damage enemies without instantly killing them.
I added some skills and items with non-lethal, low-damage attacks to help with this issue by now, but back then, it was something that bothered a few people.
The majority of the battles in the game are easy. The raids are a different story though, since they're intended to be much harder than everything else. It's not simply a level issue, level 80 is more than enough, but you need the right gear, tactic, and team composition to deal with the special ability of the individual raid boss. The same gear and tactic doesn't work on all bosses, since they have different abilities and gimmicks. You can't just "overpower" them by being much stronger, you need to adjust to how they fight.
There is a page about all the different raids and the abilities of the bosses on the wiki page at: https://renryuu.miraheze.org/wiki/Raids