Bosses are not easy and not hard either. It REALLY depends on the class you are playing and if you go with a party.
Mage is the hardest to play if you solo him. The problem is, that you heavily depend on SP which you won’t have if you dont spend thousands of silver into blue potions. So you have to use you auto attacks quite alot, which are ofc weak compared to other classes.
RIght now, I’m playing a second character and he is priest. With my lvl 5 heal I do tons of damage and heal myself and it doesnt cost much. I really have an easy time with my priest against all the bosses I had problems with as a wizard.
So I suggest you look for a party if you want to do bosses, since they really arent that hard.