Pantheon main here, but not a high ranked one or anything.
In any case, if you're up against a Panth, here's what you need to do: Buy early armor and early life regen, taking a defensive summoner like barrier and heal is great to make sure you can survive his all-in. His spear chucking is painful, so you'll want to stay defensive. Use Orianna's shield to block incoming spears if you're quick enough, make him waste mana which is his most precious resource. Zone him, you have longer range. Stay below tower and watch when he's coming at you, when he's going to throw a spear or jump on you is obvious just back off when he does. Coordinate with your jungler, if you and your jungler are on the same page, picking up exhaust can net you easy kills as Pantheon has no escape. Bait him in, exhaust and team up with your jungler to squash him. Keep talking and make sure everyone knows when he is out of your sight, especially post-6.
Pantheon only works if he gets kills early. Your new primary goal is to not let him get kills. It's not to get fed yourself. It's not to maximize your CS (though truth be told if you can do both those things that's awesome). Hell it isn't even to save your tower. Your one and overriding concern is to make sure that Pantheon does not kill you and hopefully everyone else. Once he starts roaming around (which is bound to happen) you can start trying to catch up on farm, or follow him to continuously harass, whatever you think is best in the given situation.