For easier characters like Kagura or Homura, it's not especially hard - as far as I'm aware the most important thing about getting a good rank is finishing quickly. Not getting hit, and maintaining a high combo are somewhat secondary. And both of them have really good mash combos. Try to use meter as you get it since you usually get more than you can possibly use in a level, and make use of crowd control ninja arts to mow down hordes and try to wipe out bosses ASAP - a couple strong ninja arts in succession can completely lock them out of their transformation.
Also, dodge cancelling is really useful, since it resets your combo chain and lets you reposition out of crowds where you wind up taking tons of chip damage.