Posters have mentioned how magery using pets will try to stand back and cast spells.
I took my fully upgraded Nightmare to Destard. The fight lasted a long time and took many heals. The two would stand apart and cast on each other until one ran out of mana. They separated about 5 times and did this:
I put explosive goo and magery mastery on the Nightmare. It did more damage than the GD while they were apart and fighting. I had to heal when they were toe to toe.
I'm not impressed with the nightmare but it was fun.
I took my fully upgraded Nightmare to Destard. The fight lasted a long time and took many heals. The two would stand apart and cast on each other until one ran out of mana. They separated about 5 times and did this:

I put explosive goo and magery mastery on the Nightmare. It did more damage than the GD while they were apart and fighting. I had to heal when they were toe to toe.
I'm not impressed with the nightmare but it was fun.