I want the villain to be completely in control and have everything planned like Josh Hartnett in Lucky Number Slevin or Brad Pitt at the end of Snatch.

I also want the summons to become part of you and change your appearance a bit so you just carry on fighting but with new abilities.

I don't think there would be time to summon some guy and tell him to do your bidding while you're kicking ass and taking name.