Risen appear in the chapters you have already defeated after a while. Their level corresponds to what chapter they are on, roughly. So the Risen that appear on Chapter 1 will be as hard chapter 1's enemies. So when you are on Chapter 10, you can get some quick experience points by fighting Risen on earlier levels.
Two things I would recommend. First, depending on your gald, always try to equip all your units with vulneraries, or the proper healing item that corresponds to your point in the game. Don't be afraid to use them, but also be mindful of it.
In terms of protecting healers, always keep them outside enemy range. Always utilize the enemy attack range feature with the healer in mind. I don't really ever bring more than one non-attacking healer in the first half of the game. I usually pick one out of the two (Lissa or Maribelle) and stick with them throughout the game. If they aren't leveling up or progressing to your satisfaction, remember that as soon as one of your mages promotes to a sage or a falcon knight, they can use heal. Sages are especially useful since their magic stat should be strong allowing their healing to be good regardless of their staff weapon level.