I usually take Exhaust/Flash for every Meelee DPS. Exhaust to keep up with enemy champions and flash usually to get away.

My masteries are masteries you would take for someone like mundo, PURE TANK. I like to play aggresevly so turret diving at 3 level is kinda hard with no defensive masteries...

My runes focus more on laning/soloing. Having increased mana regen with Pantheon is really important for your early game her***ing. If you dont have these runes you should go with 14/0/16(masteries)or start with a dorans ring or a meki pendant(not recomendend).

My items focus on Damage/Critical Strike. Frozen Mullet for a bit of hp and all the others are offensive.
Other good items for Pantheon.
Bersekers Greaves: Increased Attack Speed could give you an advantage early game.
Ionian Boots: With youmuus you have really good cooldown reduction.
Mercury Treads: If facing a crowd control effect team.
Guardian Angel: For the second life.
Last Whisper: The Same with Black Cleaver.
Trinity Force: I dont recommend this item but many ppl build it.

Heartseeker Strike is the main damage output from your skills. So try to max that first. Unless your playing defensive, then you will have to max Spear Shot.

Your ulti works well with another teamates stun or snare. Also you can catch up to enemys that try to get away, if you can guess right were theyre going. Really good also for teamfights because, the other team wont think of running away. Pantheons ulti deals crazy damage if the enemy is in the middle so try to make the most of that.

Overall Pantheon is a great champion that you should buy if you havent got him already. He is squishy but if you play him right he can deal tons of damage in no time.