Most people who complain about Pantheon - usually are the all-in types. He is darned fine against the Rivens, Yasuos, etc. So instead, play a tank or lane pusher. (And avoid those champs which spike on low health for extra damage. If you are waiting around for that moment, Certain Death will await you and finish you before you even get there.)
My rule of thumb is: push, push, push. For the early stages, play the slow sustain game. Wait for him to start roaming then push the lane - deny him minions. (Just remember to ping when he is missing to avoid annoying teammates!)
If you can outrange his spear (e.g., the Quinn type champ), then play that to your advantage.
Starting with early armor and some health potions is not a bad idea. At least it gets you through that early spike. Most Pantheon player tend to play "let's get that early level 2 kill", esp if they are the inexperienced types. If you have the armor, then they typically burn through their mana, leaving them quite vulnerable. (Watch for that window - and ping your jungler or friend for a gank.)
Also, take advantage of cannon waves. Helpful to get that shield down when he decides to go ham when that wave hits.