If we get picky about it we know that Celestia is capable of wielding the elements of harmony solo (see: the initial banishment of Nightmare Moon.) With Luna, they were capable of banishing Discord. Chrysalis and Tirek were cases where she couldn't do it alone, and it's worth mentioning that in the past, as she has done in the present, she has been wise enough to surround herself with competent and loyal allies such as Starswirl the Bearded.
As for why she would allow limited amounts of damage to Equestria to allow for the education of her student, it's worth remembering that if Ponyville were completely razed and everypony in it killed, it would represent only the smallest fraction of the number of ponies that Celestia is expected to safeguard and the hazards she has been facing for the last millennium are of the Discord and Tirek class, in which ALL of Equestria is at stake.
Twilight cannot be expected to go from 0 risk to "saving Equestria" in a single shot, and you wouldn't want to do that anyway lest you find out the hard way that she isn't up to the task. After years of training Celestia has been progressively increasing the risk of her lessons. By giving her student training wheel opportunities to save only a handful of ponies initially and then slowly adding ever increasing risks to the equation, when things like the Chrysalis event occur, Twilight won't crumble under the pressure of the high stakes, she'll simply take the small step up from "saving the town" to "saving the seat of government" without worrying about the fact that Canterlot has easily 10x-100x the population of Ponyville. It will just simply be "Saving folks" and the little things, like "But Celestia said I had to do this alone" won't get her tripped up; she'll have the confidence to make the decisions that need to be made, even if they're at her own expense.
Oh... And the fact that the way Celestia has been guiding Twilight's path works well in the classic hero's tale form of narration doesn't hurt things any either. It is a work of fiction after all.