I have a lvl 49 witch summoner in default, and rerolled a purely elemental damage witch. At that level, my zombies and spectres can last quite a while (lots of minion nodes/gear), but like your example... get to a boss and once your minions are dead, you have to fight the battle as a caster (and without nodes to support that).
A couple of things that could help your build - Frost Wall is really overpowered as a defensive mechanism. You can spam it, and literally push enemies back out of rooms, or block projectiles with it (Piety in her Bow form is super easy; just spam frost wall and don't even have to move). Spell Totem with your Ice Nova could also help; totems grab aggro on some mobs, and if it's spamming Ice Nova, it should give you plenty of time to kill while mobs are frozen around your totem.
But bosses are the problem; I use Fire Trap and totems against Kole for example. Hide behind a wall and he just stands there.... toss a trap, then a totem on top of the trap, watch boss walk into it, rinse and repeat. Cheap, but it works.