An opponent cannot block and kill creatures you control.
Steps
Deal combat damage to an opponent using Harnessed Snubhorn, triggering it and Breath of Fury.
Resolve the Harnessed Snubhorn trigger, returning Mirrormade from your graveyard to the battlefield as a copy of Harnessed Snubhorn.
Resolve the Breath of Fury trigger, causing you to sacrifice Harnessed Snubhorn, then attach Breath of Fury to Mirrormade, then causing you to untap all creatures you control and to get an additional combat phase after this one.
Move to your next combat phase.
Deal combat damage to an opponent using Mirrormade, triggering it and Breath of Fury.
Resolve the Mirrormade trigger, returning Harnessed Snubhorn from your graveyard to the battlefield.
Resolve the Breath of Fury trigger, causing you to sacrifice Mirrormade, then attach Breath of Fury to Harnessed Snubhorn, then causing you to untap all creatures you control and to get an additional combat phase after this one.