Destroy or not? This is the question

I consider myself a rulings-savvy person, so when i learn a ruling that I might have gotten wrong during a duel, I like to share:
Say Field Barrier is protecting a field spell, and a card that whipes spells (heavy storm) or the whole field (black rose) activates, the field spell stays on the field even though field barrier is destroyed.
Similarly, Ancient city cannot be destroyed if there's a crystal beast there as a "spell card", even though the crystal beast will be destroyed.
Also, and similarly, imperial custom on the field can protect continuous spell cards from being whiped.
However, some might claim that black rose cannot destroy power tool dragon while it's equipped to an equip spell....WRONG. The reason is that power tool protection requires sending that equip spell to the grave, and as the eqiup spell is being destroyed by blackrose in the first place, power tool has nothing to send to grave.
Exact same situation with Great Shogun Shien. To protect it, you need to destroy a six-samurai instead, but if the field is getting whiped, there's no six samurai that you can destroy to protect shogun.
Hope you find this post useful.