To be fair some bosses are easy or hard depending on if you get lucky and use the right spells against them.
One boss for example took me 15 minutes to defeat but the next time I played him I tried a different tactic and he died in about a second. Using different combos of magic drastically changes how easily you can defeat an enemy. Most enemies seem to have an instant kill combo you can use against them. As in you cast one or two sequences of magic and they all die if you know what to do.