The effect of “Half or Nothing” that halves ATK only applies to monsters face-up on the field at that time. Monsters flipped face-up, or Summoned, afterwards are not affected.[1]
OCG Rulings
You cannot activate this card during the Damage Step.[2]
The effect which halves the ATK of the opponent's monsters is still considered to be the effect of the player which activated this card.[2]
The halving effect of "Half or Nothing" only affects monsters that are on the field when "Half or Nothing" resolves. It does not affect monsters Summoned afterwards.[3]