I can't remember if I already mentioned some of this, but I plan to have Connor battle a powerful lieutenant of Satan's while the rest of the final battle is unfolding. Connor and "Louie" never really like each other, right from the beginning of book two, where Connor is behaving super bratty, and Louie is Satan's chief of staff at the Vatican. In fact, it's Louie who kills Connor's gay young lover, although I'm not sure if he does it on Satan's orders or because he knows it's what Satan wants. Connor, of course, blames Satan for the death, which triggers their battle halfway through book two (the one where Connor loses). Like Satan, Louie's zombie body is home to many demons, and he draws more to himself (into his zombie body) before his fight with Connor. That makes him incredibly powerful, though not quite a match for "boosted" Connor (after he drinks the wine made from grapes from the Garden of Eden). Since the wine only lasts so long, Connor's powers return to their usual un-boosted level, which is still very powerful, though not as powerful as Louie at that point.
The final battle between Connor and Louie will happen something like this: Louie rides out during the final battle to fight Connor one-on-one (a sword fight). Connor, still boosted, is clearly the stronger of the two, although Louie is no lightweight. Then Connor's boost runs out, and Louie begins to kick his butt. Connor's sword is knocked from his hand, and Louie keeps hitting him hard enough, Connor doesn't even have the presence of mind to draw the sword back to himself (using his telekinesis).
Fortunately, when Michael gave Connor the wine, he also gave him a small vial of the wine to carry with him, just in case. Connor also carries a vial of holy water given to him by Pope Romano in preparation for the battle. Connor, beaten to a pulp, is knocked down hard at one point, and the vial of holy water falls out of his pocket. That's when Connor remembers both vials. He uses the holy water to splash Louie's face, which causes him to back off just long enough for Connor to drink the other vial to re-boost his power just long enough to turn the fight around. It continues as a nasty fist fight, but Connor now clearly dominant. Once Connor has beaten Louie to a pulp, although both are still standing, Connor draws the sword back to himself and beheads Louie.
The other alternative is that Connor continues pounding Louie, and rather than use the sword, he hits Louie so hard with a final punch, he beheads him.
Neat trick: since Louie is a powerful demon, he uses his supernatural powers to set his sword on fire, and uses the burning sword to fight Connor, including sending flames shooting at Connor from the sword's tip. Connor lights his own sword, and they continue their sword fight. This seems somewhat like a lightsaber ripoff, so I may not do it. However, I at least have the option of doing so, because it's sufficiently different from a lightsaber that Mickey Mouse is unlikely to send me a cease and desist letter.