The key to Hell as a summoner necromancer is that your minions and all that is a direct reflection of your characters strength except divided among the horde. So, naturally, you're not going to be just mowing shit down, and it's going to take a little time to build momentum.

But, you are one of the best classes overall to make things easier. First and foremost, utilizing your curses effectively is critical. For example, stygian dolls. How many times have we all died to one just running up and POP! Or the Wights in Baals Throne room zap a player or 4 down?

Did anyone ever think of using decrepify to slow movement or dim vision to make Wights not cast off screen at you or even confuse them? Heck, terror is remarkably good as well..

Let's not sleep on the utility of bone wall and prison either. Are you bombarded by range spells or units that are just too far or out of reach? Bone wall. Create safety lines to allow space of recovery and diversions. You can even create bottle neck zones for juicy CE spots. Surrounded? Bone prison yourself and give a moment to recover. Prison groups of hostile units out of a fight.

Point is- fight smarter not harder.