In this game, you cannot neglect your defense or resists to elements. I carry around 3 rings for resists for the 3 elements. I can stand in a room full of sparks from the lightning mages and not get hurt.
Also, when you get higher level, you need to think about a specialization. Such as my templar summers a small army and burns everything (passive skills, elemental dmg nodes). I also curse them to lower/kill the mobs resists.
And sometimes you have to use the environment to help you. For example, I filter mobs though doors to kill them one at a time. You can shoot/cast through windows and closed doors.
And.... for those pesky archers, stand behind a pole or around a corner so they cant hit you.
I watched a few youtube vids from the devs on this game. They are pretty hardcore, this isn't diablo, so you have to think outside the box sometimes.