Killing opponent's Arclight Phoenixes with Massacre Girl, with Cauldron Familiar in graveyard

Question

My board: 5 lands, Cauldron Familiar in graveyard, food token

Opponent's board: 1 untapped red mana, 2 Arclight Phoenix, Shock in hand

I want to play Massacre Girl and sweep up opponent's board. With no other creatures, Massacre Girl's trigger doesn't do anything, so I need to bring back Cauldron Familiar first. But opponent has a Shock, so if I bring back Familiar and then cast Massacre Girl, they will Shock in response to my Massacre Girl and the trigger does nothing.

Here's my plan:

1. I will cast Massacre Girl.
2. Once Massacre Girl enters the battlefield and triggers, in response to the trigger, I will bring back Cauldron Familiar.
3. Opponent can now Shock the Cauldron Familiar, but since Massacre Girl has the line of text "Whenever a creature dies this turn, each creature other than Massacre Girl gets -1/-1 until end of turn" and it is already on the battlefield, it doesn't matter, the Phoenixes get -1/-1, and Massacre Girl sweeps them up anyway.

Does this work, or does this ability on Massacre Girl not matter until the trigger is resolved?

Answer 1

Your opponent can prevent another -1/-1 effect, no matter how you play it.

The clause "Whenever a creature dies this turn, each creature other than Massacre Girl gets -1/-1 until end of turn" is part of the Massacre Girl's ability, because there is no new paragraph separating the lines.

113.2c An object may have multiple abilities. If the object is represented by a card, then aside from certain defined abilities that may be strung together on a single line (see rule 702, “Keyword Abilities”), each paragraph break in a card’s text marks a separate ability. [..]

It is a delayed triggered ability that only comes into existence as the ETB ability resolves. If the Familiar dies before the ETB has started resolving, it will not create another -1/-1 effect.

603.7a Delayed triggered abilities are created during the resolution of spells or abilities, as the result of a replacement effect being applied, or as a result of a static ability that allows a player to take an action. A delayed triggered ability won’t trigger until it has actually been created, even if its trigger event occurred just beforehand. Other events that happen earlier may make the trigger event impossible.

If you return the Familiar after the ETB has resolved, your opponent can simply decline to Shock the Familiar.

Therefore, if your opponent plays correctly, you will not get a second -1/-1 effect and the Phoenixes will survive.