What I did for success:
- Dump all of your stat points on strength for the main character.
- Dump all of your stat points on magic for Nemissa, she becomes hideously powerful if you do it this way.  Your stats are capped at 40 for each attribute.  Once I hit 40, I started building agility.
- Have some demons who can run media, can revive an ally, and can cure ailments.  Always less costly overall than to use items which I always treated as a last resort.
- Install the save anywhere module.  Save points are few and far between and the game locked up on me about a dozen times.  Save at critical moments like clearing a floor or before you go into a boss area.
- Explore dungeons, rack up mag, and open up chests.
- Build up demon loyalty if they're worth keeping around.
- Trade in your mag for money.  If you do enough grindan at the beginning, you will never run out of mag.  Be sure to have enough for a full demon party though.