was debating with my friend today over who wins first in the scenario:player A had a face up bora,with no hand or set cards while player B had a set marshmallon and no hand and other set cards.player A lifepoint is at 1000,while player B is at 200.so,take a pick!as i will reveal the answer below.

Effect(Bora): If you control a "blackwing" monster other than "Blackwing - Bora the Spear", you can special summon this card from your hand. During battle between this attacking card and a defense position monster whose DEF is lower than the ATK of this card, inflict the difference as battle damage to your opponent.

Important things to take note is that the damage inflicted by marshmallon is AFTER damage calculation,while bora inflicts battle damage DURING damage step,i think by now the answer is pretty obvious.anyway to elaborate further i will include the break down of damage step:

Substep 1. Start of the Damage Step
Substep 2 Flipping the Face-Down Monster Face-Up
Substep 3 Before Damage Calculation
Substep 4 Damage Calculation
Substep 5 Battle Damage - After Damage Calculation
Substep 6 Resolve effects(EG the Trigger Effects of "D.D. Warrior", "D.D. Warrior Lady", and "D.D. Assailant" activate at this time)
Substep 7 End of the Damage Step - Monster(s) Destroyed by Battle are Sent to the Graveyard

As shown above,the victory is already declared during substep 5 of the damage step,which is BEFORE the effect of marshmallon takes place.