Mission

They have 3 Difficulty in this game for Main Story :
Normal

Hard

Hell

You unlock other difficulty when you clear one of the chapter

By exemple : Clearing Chapter 1 and 2 will unlock Chapter 1 and 2 on Hard Mode

Mission
They have 4 Type of Mission :

Story Mission
For know the story of the game

Special Mission
Gathering material for upgrading your Unit or Lvling them

Character Story Mission
The Story of the Character