You know, the people and builds who roll you over. Play drow stsf dagger in battle, play air/off shifter, play batadin. Play that levelsitting sleep monkey. Then, after 300 hours of each at hero, you'll know how to beat them. As for preps, stone skin and protection is very easy to come by, aura a bit harder, but ask ic and you will learn. Seek to be better, but I think it just comes down to people who spent literally thousands of hours so they know the tricks. Still, they are still beatable. Play the stock power builds and stick to condeath and you will learn too. Or not. Roll a herald neutral healer and not deal with it all.