Opponents with creature cards in their graveyards cannot block creatures you control.
Opponents have at least two creature cards in their graveyards.
Steps
1Deal combat damage using Shark Shredder and the creature with Breath of Fury attached. Shark Shredder and Breath of Fury trigger.
2Resolve the Shark Shredder trigger, putting a creature card from the defending player's graveyard onto the battlefield under your control tapped and attacking.
3Resolve the Breath of Fury trigger, causing you to sacrifice the creature it is attached to, then attach Breath of Fury to the creature put onto the battlefield in step 2, then causing you to untap all creatures you control and to get an additional combat phase after this one.