@Naruto
Basically what leveling up changes are Life Points, attack and defense: while attack and defense have a linear increment, Life Points increment a lot... so an extra level up makes you kill an enemy in less attacks and you can resist more hits before you need to use a blood vial or before you die.
Since i studied the parameters of the enemies according to the progresses in the game and more or less i know which level a player will reach in a certain area, most of the times the player will be at the expected level or one level below the expected (on purpose), unless the player gets stuck and goes in circle for a while, in that case it will get the expected level or one level above the expected. Since this structure is quite rigid and Elyon caps at level 10, one level below the expected means you're underleveled; one level above the expected means you are overleveled.
To make a long story short one level up helps a lot and the more you level up, the more that's true