You cannot activate "Strike Ninja"'s effect during the Damage Step.
"Strike Ninja" is a Multi-Trigger effect so you can chain it to another Spell Speed 1 or 2 effect and can activate it outside your Main Phases, even during your opponent's turn (but not during the Damage Step).
You can only activate "Strike Ninja"'s effect when "Strike Ninja" is face-up on the field.
Removing 2 DARK monsters for "Strike Ninja"'s effect is a cost.