To fight against the Philly shell and win, you'd need to use jabs and feints to create openings in their defense. Use your jab a lot to poke the Philly shell fighter to open their body defense wider before cracking them in the chest and ribs. You can also make use of the low body jab to force them to bring their guard tighter and shoulder down before catching them with an overhand cross. The cross needs to come over their shoulder where they can't deflect it and get them right between the eyes.