Slight correction needed here, by this point Harry has done his Ghost Tag Team rematch with the soul sucking thing, and has actually not only regained all his former power, but also boasted it with some of the former Warlocks power. (The former Warlock/Now former Ghost magic was pretty much all destructive but that was okay, Harry was in a very destructive mood)He also managed to cook a courtyard full of Red Court Vampires by calling up a fire storm that had flames 40 to 50 feet tall mind you this was after a power eating specter from hell made a snack out of his soul and his powers leaving him a mere shadow in terms of his power levels. Harry also tossed a car on top of another wizard by the name of Cowl.
Added to this, the fact that it was Halloween, the veil between the living and the dead was at it's thinest, and further weakened by the actions of the Vampires themselves. Harry called upon the Countless number of dead under his feet for his little show with the Red Court Vampires.
On the other hand this was also early on in the books, and Harry has been getting more powerful as the books go on, getting more control, and getting just damned sneakier. This s the guy who worked out that when he was fighting the Ghost in the dream setting he needed some extra help, and so created his own Ghost to back him up after all...