Why is this a thing?

Recently I've noticed that whenever theres a pantheon on the enemy team they win. This one pantheon plays support and feeds the first 4 kills on purpose, and then he still one shots my entire team 10 minutes later like wtf is that. I was playing another game laning against him I'm tower hugging he just walks up throws a spear at me and then i lose 200 hp and im level 4 1000hp and then he takes no damage because it his passive shield and then he just repeats untill im half hp then he just stuns from mad far and then ignites and im dead. I feel like this champion is way too strong with lethality and he should be nerfed or something because its just unfair, he just shits on the top laner and then ults mid or bot lane and just gets fed easily because he is broken like that.