Wild levels of creatures are decided by 3 factors,
DifficultyOffset & OverrideOfficialDifficulty.
Core mods that remap levels.
Some exceptions exist, like Wyverns, TEK creatures, and RockDrakes, they can always spawn at a few levels higher. The same thing applies to core mods that alter creatures' levels, and core mods that alter the difficulty, even though the latter should no longer be used since we can change difficulty ourselves now.
DifficultyOffset must be a value between 0.01 and 1.0, the higher the value, the more difficult gameplay. This does not control creature levels by itself but rather controls various things like chances for high-level wild creatures, loot drops qualities, taming efficiency, etc.
OverrideOfficialDifficulty controls the max potential level a wild creature can be. Each increase of this value generally represents 30 potential wild levels. If you are playing on TheIsland with DifficultyOffset=1 and OverrideOfficialDifficult=10, the max wild level will be 10*30=300 and the minimum wild level will be 10.
These settings should be placed in your GameUserSettings.ini.