If Mardu Siegebreaker leaves the battlefield before its "enters" ability resolves, the target creature won't be exiled
Once you begin casting a spell from your graveyard, it immediately moves to the stack
When you do, target creature defending player controls gets -X/-X until end of turn, where X is the number of artifacts you control plus the number of artifact cards in your graveyard
The field notes art shows a Quandrix student sketching out how to create the copies with fractal magic
I did make a section for decks that will counter this, but it will mainly be DialgaChomp
So stop dragging your knuckles and turn the pageits time to get down to monkey business." Bihldorff, Nate, Jason Leung, and Drew Williams (1999)