Just like Harry casts fire spells without his blasting rod a few times and complains about how much harder it is.
Plus, a better mage can make better focus items, so by the time you've got to that level you can make staves and jewelry of legendary quality.
A Flock of Sheep.
A Pack of Wolves.
An Inconvenience of Heroes.
The Blackstaff seems to give McCoy some degree of immunity to the negative psychological/physiological effects of breaking the Laws of Magic, in exchange for at least a temporary physical corruption. OTOH, Granny doesn't suffer any sort of negative side-effects from doing things that would arguably count as breaking the Laws of Magic, so if that still holds true, then she's at least got that much open to her.
As for taking on an army, the last time she faced down with a horde of vampires, she brought a phoenix with her, after screwing around with their minds by possessing her own blood. That should help in the Changes situation (if it ever came to that, which it won't because that situation isn't remotely applicable to her), assuming she can get a similar set up.
OTOH, if she talks to Uriel and he manages to convince her that there is a benevolent God out there, then she will probably jump into His service and go full out Knight Templar and have to be put down by the Council and/or the Church, going by her comments to that effect in Carpe Jugulum.
Fox Mr. Sinister (holding Rogue's chin): "So beautiful, yet so strong."
Fox Wolverine: "Well, I always did think I was kinda cute. Nice of you to mention it."
Official Board Ostrichizer