If you play in defensive mode, I would definitely go with ?/Electric. Great heals and buffs (that have a chance to affect your character as well) and a positional forcefield that you can drop on the spot you are fighting from. (I have had the most success with ?/electric)
I generally target through my most powerful henchman, run up into the middle of the enemy group, drop the forcefield and keep buffing and healing henchmen with a stray attack here or there.
What to pick for the primary?
I'm a character conception player, so I'm limited to what I think makes sense. I'm not trying to force a reasoning.
With the whole Dr Frankenstein thing, I have no problem going with /electric for Necromancy.
Robotics? /electric make sense to me as well.
Demons? /electric makes sense for a henchman-less mastermind, but then I'm generally running on teams and using the team as my "henchmen". The character jumps in to the location as the tank and uses them as the "most powerful henchman" in the tactic.
The other henchmen/animals don't really seem conception-friendly as far as /electric is concerned.