Cannon Soldier, Black Ptera, and Ultimate Offering. This is similar to the Dark Magician of Chaos/Cannon Soldier/Dimension Fusion/Spell Economics OTK but this one is actually legal.
Conditions: Your life points cannot be over 400 points lower than your opponent's.
Procedure:
1) Have Cannon Soldier and Black Ptera on the field.
2) Use Cannon Soldier's effect and tribute Black Ptera. Black Ptera will return to your hand due to it's effect.
3) Activate Ultimate Offering and summon Black Ptera.
4) Repeat steps 2 and 3 until your opponent's life points reach zero. You Win!
Side note: Also, if needed you can tribute any other monsters you have on the field to finish off any straggling life points your opponent has