I'm playing Jax against Pantheon right now -- any tips? Should I just play passively until 6 and burst him down?

Early game you will get raped. Just try to farm and get big mid to late game when you start scaling like crazy and Pantheon falls off. I would suggest starting with Cloth Armor and 5 Health Potions and to stack armor. Get Ninja Tabi, and stack Doran's Shields.

if you're in an experimental mood you could see if leapstriking to a minion cancels the stunleap from panth if you do it while he's in the air.