Dakkodango Translations

This is by far the biggest problem I have.

My red spirits (Orpha, Nanaru and Himika) have very crappy defense so using them to defend on hard mode and above is suicidal.
Himika loses mind because she kills a lot of units without gaining any mind, and Nanaru has terrible attack skills. Orpha is not as much of a problem because she can gain mind from scenes and she doesnt lose any from using support skills.

So how can I raise their mind without capturing town, killing dragon and obtaining mana crystals?

actually, in very hard life far outstrips physical damage, so even orpha can take a heaven's sword (no buffs) and live. Nanaru and himika have about 60k life at max level in very hard and do even better. You likely face more problems in hard difficulty. To resolve this, try buffing your red spirits when they defend with skills like gaia's bless. Orpha is a main heroine, so if you complete a mission in ss timing she gets 10 bonus mind. If all else fails, defend with a red spirit and use revive to bring them back at the end of the battle. Esperia learns revive at a mere level 26 so you should easily get it, unless you overwrote it with another skill.

Anything not named Faren/Orpha can tank in hard mode.
Anything not named Faren can tank in super hard mode.
Exception is Esperia and magic. They just don't go together.
And the best tank in this game is Eternal Orpha, go figure that out

Actually, Orpha is the hardest girl to keep on LOW mind, and red spirits in general are the easiest to max mind in higher difficulty mode

Try wasting a turn with buffs. I imagine doing a Holy or Gaia's Bless would make Himika and Nanaru very tankable. Also, mana crystals don't change in hard/super hard mode (10000 mana), and when you get to level 50 or so, they're chump change. So you really do want to put them towards mind instead.