I've just finished the Ent on earth and am now searching for the undying king. Tbh, I met a guy from the states who is a lord of death on this game who helped with the ent, and has been taking me places in order to get better stuff. Otherwise, I would still be struggling.
Yeah the game is hard, but really good and great in coop. I do agree with you that the bosses feel that they are handled better coop. You can have at least one person thinning the minions whilst at least one person attacks the boss.
One other thing is the fact that the starting weapons and armor are absolute dog shit.