Difficulties
The three difficulty levels found in Trails in the Sky are Normal, Hard and Nightmare. Earlier versions may only have Normal and Hard difficulties. While these difficulties don't affect the game itself, it does affect the combat, and the tactics you need to employ to survive.
Enemies will be stronger, cast more destructive arts, have higher defence, and possess more health.
Normal
The "Normal difficulty" is the most accessible version of the game. It doesn't require much strategy to defeat the enemies, but you are expected to use Arts and plan around weaknesses. You are required to use Crafts in some Boss Battles.
It is recommended that you equip one of each kind of Quartz to ensure you aren't caught unprepared. Equip the best weapons and armour you can afford and obtain for each area.
This version requires very little item use as healing Arts will often be sufficient for survival.
Hard
The "Hard difficulty" demands additional strategy during encounters compared to normal. Changing battle positions in Tactics is expected to survive early and more difficult battles. The use of Arts and Crafts is heavily required to be victorious. Attacking enemies from behind is recommended.
Some experience grinding may be required to increase your level, and opening Orbment slots early in the game to allow for casting multiple varieties of Arts. Finding the best Arts for each enemy is required when encountering enemies at lower levels.
Converting Sepith into Mira is needed to afford ingredients for use in recipes. Using recipes to make items will heal wounds while bolstering your abilities. It is better to use offensive Arts and use Items to heal to conserve your EP.
Nightmare
Nightmare is the hardest of all the difficulties. Using Tactics to re-arrange your party in the battles is essential for surviving. Putting everyone in the rear of the battlefield is needed for most battles. The use of Crafts and Arts are also required to be victorious in every fight. Attacking enemies from behind is recommended.
Fighting Shining Poms are required to increase levels and gain Orbment slots early in the game. Fight regular enemies until your CP is 200, then hunt for Shining Poms to increase your experience and Sepith. Focus on the enemy's weaknesses and use your most powerful Arts.
Extra Mira is required for items and ingredients. Convert your extra Sepith so you can afford the best weapons and armour as quickly as you can, and also buy the latest recipes and ingredients. Use your EP for offensive Arts and food to heal and strengthen your party.