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.
Netrep Rulings:
If “Strike Ninja” attempts to return to the field when all monster zones are occupied, “Strike Ninja” is destroyed.
“Strike Ninja’s” effect only applies while “Strike Ninja” is face up on the field.
The effect of “Strike Ninja” returning to the field after being removed by its own effect is not a Special Summon.
GoatRulings.com Rulings:
Q: Can “Strike Ninja” return in the End Phase if my monster zones are full from having activated “Return from the Different Dimension” previously this turn?
A: Yes. Since both “Strike Ninja” and “Return from the Different Dimension” will resolve in the End Phase you can choose in which order this will happen. You can choose to resolve “Return from the Different Dimension” and then resolve “Strike Ninja”. In this instance “Strike Ninja” will return to the field. If you choose to resolve “Strike Ninja” before “Return from the Different Dimension” then it will be sent to the graveyard.