Well, you should be experiencing what we describe now. In my case, up to lvl 10-12, the game was challenging, after that not at all. It also depends on the path you're following : if you severely underleveled, you might experience the difficulty drop a bit later, I guess.

However, I'm a bit surprised by your AI behaviour. I would say that in most cases, enemies target summons with high priority even if they are far and take damage doing so...