Assuming we agree that new characters need to be introduced fairly often to make up for all the delightful murder, it shouldnt be too big of a surprise when some of them turn out to be competent. There's only so many bastions of peace untouched by the zombles where we can believably find 'normal' people to be all incompetent and make bad decisions so they can be saved by Rick.
Anyone surviving that long, let alone on their own, is going to have figured out how to survive and unless Michonne is bouncing off walls, leaping off building and doing other silly bullet-time moves, I dont have a problem with it. She comes across as eminently practical to me (as long as there's some serious psycological scarring to go with it).
Disclaimer: I havent yet watched the second half of the season as I was waiting to watch it all together so might feel similarly enraged when I actually see the entrance.