Is Pantheon UNSTOPPABLE?

Pantheon is meant to bully an opponent in lane, but he's offset by terrible mana efficiency and a lack of escape if he dives. A good idea would be to get your jungler to gank him (if your jungler listens at all) after he uses his W.

If you can't get that, rushing some armor will definitely help. Don't get trapped in the mindset that you HAVE to go Black Cleaver or Trinity Force first; that's stupid. If he's causing you a lot of grief or has pulled ahead, going for offense is just going to lead to you feeding him because you'll die just as quickly as before (if not faster). A Chain Vest can go a long way early on, even if you don't have all the health to back it up.

If he is the singular champ who gives you most trouble top, you could always just ban him away. If that gives you a bit of comfort, it's worth.

Could always practice and play champs who counter him. Olaf, Shen, and Darius can weather his combo and dish back more.

Pantheon is meant to win lane. That's straight up what he's good at. However, he falls off hard late.

Pantheon's win condition is killing you so hard that he can snowball the game. Your win condition is surviving so that he can't do that.

Just play it safe, and you've already 'won', even if you end up a bit behind. You'll most likely need to build defensively or wait for jungle assistance.