This card itself can serve as one half of the cards that need to be in danger of being destroyed. This card can then be triggered, leaving only one other of your cards exposed should your opponent negate this card's Effect or interfere with its resolution.
The conditions for this card's activation is not bound to your opponent's moves, you can also activate a card of your own to destroy your own cards like "Dark Hole" or "Two-Pronged Attack". Use this with your own "Sacred Phoenix of Nephthys" and you will get it back next turn.
You can use a card to destroy your own cards, then negate it with "Starlight Road" if your goal is to easily summon "Stardust Dragon". This trick can be useful in an "Assault Mode" deck.
Remember that the second effect is optional, so you don't need to have a "Stardust Dragon" in your Extra Deck in order to activate this card or to make best use of it, although it is usually advised that you do because it can make summoning "Stardust Dragon" easier.
Combine this card with "Tyrant's Temper". This card must be triggered against destruction effects in order to Summon "Stardust Dragon". While "Stardust Dragon" negates destruction, "Tyrant's Temper" protects your monsters from Traps.
Important to note, the "Stardust Dragon" summoned by this card effect, cannot revive itself via its own effect or revived by any card effects, because the "Stardust Dragon" was not Synchro Summoned properly.