Concise Rules are.
If the attack is Melee or Ranged X - use ATT
If it is Magic X - use Will
If it is Missile - Use Dex
If the character themselves has Magic or Missile in their 'middle stat bar' (the text between the picture and the special actions) then you can choose to apply that attack type to any of their attacks listed below. i.e. The Glimmerdusk Ranger's Sparkle Burst can be Missile 6 using her Dex.
Note: You don't have to make a Magic or Missile attack if you don't want to, all characters can always make melee attacks using their ATT stat. This is useful if a non-melee character wants to hit a Boo Booty (who are immune to non-Melee attacks)
Thank you, this was very informative!