As a melee champion who relies on mostly auto attacks? if i build damage, he does more than me and blocks enough of mine, if i build defense, he just goes through it. what am i supposed to do? should i just pick champions that have spammable abilities with low/no cost? is there anything that i can do if i don't know i'll be fighting 
(TB, Blind pick, etc.)?
or survive until late game where he is utterly useless
Sustain works, depending on the champ in question. For example, if Garen starts with cloth and 5, Pantheon will be hard pressed to be a relevant threat towards Garen even if Panth spams his Q unless Pantheon hard engages with his Aegis. The thing is though, Garen wins in straight up fights. If Panth engages on Garen because Garen's sustain is able to get past Panth's poke, Garen is likely to just turn around and Auto-Q-E Panth to death.
Shen and Jayce have good pokes.
Ask your friendly Neighborhood 
to rip him a new "BLEEP"
Honestly I find 
harder than pant to face during the laneing phase
Frozen heart is actually a bad item against pantheon. Great against auto attackers like Yi or Fiora, but panth is more of a bust AD caster. A better item would be sunfire cape because it provides more Effective Health so you can't be burst down and helps supplement your damage with an unblock able aura that his own passive cannot mitigate.
Edit: maybe bad item is a bit harsh...the AR helps and the CD may be useful. But it's less ideal than Sunfire for surviving a boxing match with the mantheon.
Farm undertower, You need to pick up armor/hp or armor/regen. If you still have hard time then you need to just farm the best you can without dying. panty isnt stupid to dive you under tower so hell try to poke you a lot... even under your tower because of his shield. as long as you can clear waves well and maintain your hp with armor/regen/hp then you're doing ok. if tower is going down then let them have it. better than dying and lose tower. i believe that his shield doesnt block cc so you can try and cc him as he go deep into tower to spear you.
I just play Irelia vs Panth.
