The attacking order of creatures is decided by when they came into play. The creatures that were summoned earlier will attack sooner than ones recently put into play.
Targeting Immortal or Burrowed creatures is a very strange matter to me. They are subject to shield effects (damage reduction, freeze chance from Ice/Permafrost Shield, time bubbles from Procrastination/Turtle Shield, damage from Fire Shield/Buckler) but they can't be affected by spells.