Welcome! My two cents (and the great thing about this site is that you'll get plenty of opinions) is that I like one campaign per domain. For me, it's really about seeing what's happening within mysite - and, using your example, www.kibin.com/essay-editing is a page within your kibin.com domain.
Focusing on just a subfolder, like your essay-editing section, just limits your overall visibility into what's going on within your site. Personally, I look at the big picture then whittle down into the subfolders and whatnot.
Why do I have that preference? The pages have to live within the overall domain - I look at the relation between them, how they each rank, where I may cannibalize one vs the other, what page ranks well, what doesn't. This is what Moz campaigns are great for.
Moz will tell you which pages rank for which terms. You could learn that, again using your example, a different page is ranking for "'essay editing" than the one you seem to want, www.kibin.com/essay-editing.
(Although, having just looked at your main domain and your essay-editing page, they look exactly alike so there's duplicate content and possible cannibalization going on - or something isn't functioning properly and the page won't load; I didn't dig into it.)
Now, since you can set up multiple campaigns, there's nothing to stop you for doing both, either. You can always try it that way and see what data you see and then tweak as you'd like.