Control Dudley of course.

"Dudley, I need you to be my lab rat again. I can make you fall for Marge, but I will be gaining the experience from it as I will be controlling you."

"Of course, Master Harry. I apologize for making you do extra work and as my punishment let me streak across the streets naked and humiliate myself."

"Uh, not needed at all. It will affect our plans either way. Your penance shall be that you will never know how you made Marge fall in love with you."

"You are forgiving, Master Harry. I thank you... Now do as you please with this tool."

Before you assume Dudley, you transfigure his tiny cock to match your size and then start possessing him...

You take over him and you begin marching towards the Dursley Household for the final feat.