In many mmorpg, monsters are not a big challenge, what do I want to know and if monsters will attack us in different ways? or will they just hit the player repeatedly?
My opinion is that the monsters have to be balanced, if they are too easy to kill you won't feel the challenge, if they are too hard to kill, you will be frustrated by the constant dying.
They have to be enough hard for you to feel the challenge, but not to get you frustrated.
That goes for every class, it wouldn't be nice if, for example, the Warrior can kill everything without a challenge, but on the other hand, the Mage dies whenever more than 2 monsters are hitting him.