Well I spar against a southpaw he's a pro cruiserweight, he doesn't always fight southpaw he apparently likes fighting me that way because it makes me throw my left hook less and he uses his straight left more.

As far as fighting a lefty doesn't change much of your tactics or anything just makes you feel a little awkward in there, and head butts are always dangerous when fighting them. You can still throw the cross over the jab and still land the hook ( when you get the angles ). Not ALL lefties have Winky's defense you can open up the head or the body by switching up what you attack. Just try not to be predictible like you would do in any other fight.