The simpelest way to do it is to have 3 enemy controlers in your deck and some monsters. He will summon his Ra. Then activate the second option on enemy controller and take control of his Ra. Use the effect of Ra to destoy his cards and then atk ko with the 4000 atk of Ra.
Not fully waterproof but worked on my first try
My way was a little more complicated than the enemy controller way.
I used Mystic Box to destroy Ra and send a weak, Normal monster to his side of the field, then conducted normal battle, using cards like Dian Keto to keep my LP up, then finally used his Lava Golem against him and summoned it to his side of the field to get rid of the last of his LP.