Once you have the battle tower it becomes pretty easy to jump back to your necromancer to switch parts.

You can not control the creature, or give it potions, etc. If it is getting low on health and you are expecting a challenge ahead, un-summon it, wait for the cooldown time, and re-summon it. With Mana Efficiency at level 5, summoning it only take a quarter of you mana, rather than half.