0 - Absolute superiority. The enemy can not do anything. (very very low dif, like Acno vs Happy)
1 - Absolute oneshot. The enemy may try something to get done.
2-3 - The obvious superiority. You can oneshot depending on magic, but the opponent is able to fight back.
4-5 - The marked superiority. Very rare and cheat oneshots are present, if the enemy is weak to character's magic. The winner is obvious, but it gives the enemy a good fight.
6-7 - Without the obvious superiority, the opponents are equal in the general parameters. There is a slight advantage, decent fight. (Mid dif)
8 - Without excellence, possible slight superiority on the part of the enemy, victory is not easy way. (mid-high dif)
9 - Weak superiority of the enemy, oneshot is possible depending on the magic. The winner is unclear. (high dif. Like Laxus vs Hades on Tenru but with Laxus' win)
10.The superiority of the enemy, oneshot guaranteed, save perhaps only a miracle or cheating. (like I said, Jellal can oneshot Natsu with AB but if Natsu survived it somehow or Jellal will never use AB than Natsu with extreme dif win)