All dungeons will be ranked from F to SSS as so:

Rank F - Level 1 - 5 Monsters, E Rank Boss Monster

Rank E - Level 6 - 10 Monsters, D Rank Boss Monster

Rank D - Level 11 - 20 Monsters, C Rank Boss Monster

Rank C - Level 21 - 30 Monsters, B Rank Boss Monster

Rank B - Level 31 - 40 Monsters, A Rank Boss Monster

Rank A - Level 41 - 50 Monsters, S Rank Boss Monster

Rank S - Level 51 - 60 Monsters, SS Rank Boss Monster

Rank SS - Level 61 - 70 Monsters, SSS Rank Boss Monster

Rank SSS - Level 71 - 100 Monsters, Death Rank Boss Monster

Death Rank - Only one dungeon of this rank and it is where the dragon came from and contain many strong monsters that had an average level of 125 with some even being as high as 150

The strongest monster ever to appear was a dragon at level 200 that was defeated after years and at the loss of MANY high level guardians ranging between 2-5000 and no one knows the highest a human level can reach as the highest level human is 90 and only reached it after 50 years even with devoting most of their life to training and hunting after unlocking their class early due to being from a strong and wealthy family.

The way the system works is that the system collects all your actions into what you could imagine as a folder of sorts that when the first level up is achieved is opened up and all your actions are collected and analysed to find what the system thinks of as your 'ideal class' so if someone was to practice magic enough (which everyone can use from age 10) then upon their first level up they will be given the mage class, all people can access magic at age 10 but all they can do until their first level up is control the flow and manifest a ball of magic which is considerably hard for most below level 1 and they can't cast any spells, or if someone practices martial arts they may be given the fighter class or if they're skilled enough maybe the martial artist class. At level 10 people are once again given the option for new classes, still related to their actions, sometimes there may be an advancement of their current class but it's mostly just a different class however once this is chosen then they can no longer change again.