Not Updated For Current Season
(Special thanks to Tbore and DEWO for their Pantheon guides)
Pantheon is one of the best mid-game heroes...and with a high AD set you can deal high damage with your spear and heartseeker strike.
Pros of this item build
Ridiculous burst with heartseeker
Consistency...Manamune will allow you to never go OOM and no reliance on Sword of the Occult bonuses
Using your spammable spear gives you more AD!
Cons of this item build
No Sword of the Occult which can be potentially amazing
Health and defense is very low for late game (consider the item "Frozen Mallet" if you want more health and utility)
Attack Speed is lacking
Pantheon is a simple hero, he has 4 abilities that are all cooldown reliant, and he bursts enemies.
For early game spam your spear on creeps with low hp, and if you're able to, spam it on opposing laning champions to harass. Use a W + autoattack + Q combo early on to inflict decent chucks of damage and save heartseeker strike for opportune moments since it costs a lot of mana.
For mid game your ultimate is often your initiator, once you hit level 6 you should check for enemy champions below 1/2 health. Your ultimate takes practice to perfect but I have a few tips:
1) Try to aim slightly behind the enemy laning champion(s) so that they have second thoughts about running back towards the center of your circle.
2) If you have Tear of the Goddess (only 605 gold with Meki Pendant) you should have enough mana to use this away from the fountain and still use the rest of your skills.
3) If you have second thoughts about about performing your ultimate, you may cancel the cast while it is still charging and have a 10 second cooldown to start charging it again. Note that you may not cancel it after it's done charging.
4) It has a cast-and-jump-and-land time slightly less than recall. Use to your advantage if you can.
Once you ultimate (or gank from the bushes), W+E+autoattack+Q, if the target isn't dead you can exhaust or ghost or flash or whatever it takes to use spear and autoattack again, but don't chase too far. Pantheon is an excellent mid game hero and you should be able to kill many champions with this simple strategy.
Late game is when pantheon takes a back seat to tanks and carries, he still has great burst potential but it is more difficult to pull off. You are very squishy at this point and the enemy team will focus you if you have poor positioning. Since your W is an ability that sends you towards your opponent, using it in team fights is not often a good idea. Instead, you should try to flank the enemies when they are distracted by your allies and run up and heartseeker strike into the crowd. You should have some range between you and your enemies with this strategy which allows you to back off if necessary. YOU SHOULD ONLY W+E IF YOU ARE CONFIDENT THAT YOU WILL NOT DIE.
Oh, and keep spamming that spear! It scales with AD like heartseeker. :)