Yup, you have to do everything in one go, time won't be enough if you plan to walk around the areas twice. This means that you have to strike most of the impossible enemies where you meet them first. Beating such enemy gives more satisfaction too
I had 2 enemy heroes in the land of the dead, one with 60+ giants, and one with 600-700 necromancers, both of them impossible, and quite hard to beat. Another hard thing that I did was upgrading marshal's baton at lvl 16 iirc.
For spirits, to level them that much, you need several things - first, the corresponding skills, and being warrior helps too; second - you should use each of them every battle; Lina is quite hard to level - so most of the battles you have to open either with chargers or ice thorns.