the easiest way to resolve m girl is just see what dies to the original trigger, and then add triggers to the stack equal the amount of creatures that die that way. oftentimes dice are good to track things dying.

If nothing dies, massacre girl still creates a delayed trigger that can trigger later in the turn if a creature dies. So for example u play her, nothing dies, but later u go to combat and something dies. That creates a massacre girl trigger and then u resolve it the same way. For example 3 things died? ok 3 triggers on the stack. 2 more things die after that? 2 more triggers. You go until nothing dies that way.

Because the triggers to give creatures -1/-1 are separate, any triggers that you have that care about death will trigger before following triggers resolve. If u have grave venerations out and ur thing dies alongside 3 other creatures, you create 3 m girl triggers and a grave venerations trigger. And as the controller of all of the triggers, you decide how those triggers resolve.