When Heartless Hidetsugu is tapped, each player takes 10 damage, which is half of their life. Gisela doubles the damage my opponent takes, so that 10 becomes 20. Gisela also halves the damage that I take, so I take 5 instead of 10.
Is this correct, or am I missing something? If this works, the Heartless Hidetsugu will always kill opponents with even life totals while Gisela is in play. If they have an odd life total, then they will be reduced to one life instead.