Well, that are some breasts you try to go against!
I believe that punching is generally the most painfull and creates most fear in your opponents head. Still, with breasts of this size, thats hard.
To illustrate that. Check these two boxing fights:
http://rapidshare.com/files/386959253/clip1_fv113.avihttp://rapidshare.com/files/386959563/clip1_robinvstina.aviIn the first, Tina attacks a large breasted girl. It hurts some, but the punch is -so to say- absorbed by her breasts. Large breasts are made of a lot of fat. That is a non-sensitive. On the receiving end, Tina is not so happy. This punch comes in hard an grinds her glands against her ribs. That almost stops the fight.
As you probably know you can feel the difference if you jab an opponent in the chest (w/o protector and not-to-heavy gloves).
With the chest of your opponent, I think of punching square on the nipple. Or go for more "surface damage" e.g. pinching. Obviously the nipples are most sensitive. But do not underestimate how much a pinch of the upper sides (between the nipple and the shoulder) hurts.
Good luck to you.
plz