The problem is that you typically want to use some other abilities to help you control the fight. Some other abilities also deal significantly more damage (usually comparable to their mana cost) or have bonus effects (healing, etc.), which would be haphazard to ignore. Unfortunately, I haven't played Torchlight II yet (other than my time in the beta), so I can't give you specific abilities that would help you.
EDIT:
I did play the original Torchlight quite a bit, and to give you an idea, I put Summon: Skeletons and Heal All on my pet's auto-cast. My character used an elemental resistance spell along with an attack speed increase spell. I also had numerous abilities that allowed me to do things like perform area of effect (AoE) attacks or dash around to avoid bad situations. I was playing a user-created class, so stating ability names wouldn't be too helpful.