You have to look aout at wich LvL Range the Mobs you attack are.
Green 1 Star upt o 4, or 5 if I remember correctly. The more Stars the harder to kill and the harder they hit.
And you have to get better Armor to survive
Simple as that
With the root spell its almost alike, some mobs are immune, but are far more affected in turn by say Ice, or Slow. In other mobs it may be just the other way´round.
Nothing is quite like the other here
And that´s what makes it so exiting. Hope it does not frustrate you too much.
Acridiel