It makes sense when you say you had success with the lead ribs. The punches you want to avoid are hooks to the rear ribs cos it brings you too into deep to be countered with the right hand. Basically they are looking to counter with a rear straight or lead hook. I don't know about the best way but the safest way is to attack the lead side of his body/head and move/slip/roll to the outside of his lead hand afterwards. Don't overcommit and pick him off with the jab and use convincing feints. I may be wrong but the philly shell user is waiting for you to lead so he can time this and answer with his own counter. Why give him that? Try to feint him into leading himself and hit him as he is coming to you as in that moment he won't be in a defensive philly shell.