These TCG rulings were issued by Upper Deck Entertainment and have since been deemed unofficial by Konami. They were previously considered official and can still be correct, as long as they follow the official gameplay rules, or unless Konami has issued a ruling that says otherwise. References: 
Gravekeeper's Assailant: If "Gravekeeper's Assailant" attacks, and activates its effect to change the battle position of the opponent's "Spirit Reaper", then "Spirit Reaper" is destroyed by its own effect. Then a replay occurs because the potential attack targets on the opponent's side of the field were changed. In this case, "Gravekeeper's Assailant" can attack again, and can activate its effect again.
Neo-Spacian Dark Panther: If you target "Spirit Reaper", the "Spirit Reaper" will be destroyed after "Neo-Spacian Dark Panther's" effect resolves.
Ninja Grandmaster Sasuke: A face-up Defense Position "Spirit Reaper" will be destroyed by the effect of "Ninja Grandmaster Sasuke". "Ninja Grandmaster Sasuke"'s effect does not target. So if "Ninja Grandmaster Sasuke" attacks "Spirit Reaper", "Spirit Reaper" is destroyed by "Ninja Grandmaster Sasuke"'s effect, not by "Spirit Reaper"'s effect.
Staunch Defender: When you select "Spirit Reaper" or "The Reaper on the Nightmare" as the target of "Staunch Defender", resolve the chain, and after the effect of "Staunch Defender" resolves, the selected monster is destroyed. A replay will occur, but the opponent can no longer attack because the selected monster is gone.
Last Turn: The effect that reduces Battle Damage to zero applies only for the special Battle Phase, not for the normal Battle Phase.
Example: A player activates "Last Turn" and selects their Defense Position "Spirit Reaper". The turn player Special Summons "Enraged Battle Ox" and attacks in the special Battle Phase, but no damage occurs. However, during the turn player's normal Battle Phase they can attack again, and will inflict 1500 points of damage due to the effect of "Enraged Battle Ox", so the opponent's Life Points will be zero and the turn player will win.
Regenerating Mummy: "Regenerating Mummy"'s effect will activate during the Damage Step because of "Spirit Reaper" or "Don Zaloog".
↑Konami Judge Program Rulings are considered unverified, as their source(s) are not publicly viewable. They are not confirmed as applicable to the TCG until they have been officially published by Konami.
Out of Date
↑The latest print of "Gemini Imps" has Problem-Solving Card Text, which does not specify that it cannot be activated during the Damage Step. Since this card negates activations (which allows cards to be activated during the Damage Step), it can be activated during the Damage Step.