You cannot activate
"Tragedy" during the Damage Step.
You can activate
"Tragedy" after an Attack Position monster is changed to Defense Position by a card effect, such as
"Enemy Controller" or
"Stumbling".
"Spear Dragon" changes its battle position at the end of the Damage Step, so you cannot activate
"Tragedy" at that time.
"Goblin Attack Force" and
"Giant Orc" change their battle positions at the end of the Battle Phase, so you can activate
"Tragedy" at that time.
Missing the Timing: The position change must be the last thing to have happened. If
"Zero Gravity" was activated as Chain Link 2 or higher, you would not be able to activate
"Tragedy".