Not Updated For Current Season
try to spam your spear as much as possiple. you will get enough dmg done doing this. you will need a good laning partner, sokara and sona are perfect to have around. they will heal you like crazy, so you can stay in lane. try to harass with your spear, you can get fb if your teammate has a stun/slow. as you can see this build is set up pretty defensively, this can really save your *** as pantheon can be killed in the blink of an eye if he doesnt watch what hes doing.
if you are up against spellcasters: take chalice of harmony first. and later get a banshees veil, this will give you a second shield :) 1 for dmg and 1 for spells. that gets you pretty safe, and allows you to throw your 500 dmg spear a lot. also, hitting champs for 800-1000 crits is nice too. And counting trinity force this can be alot more !
try to spear shot last hit minions, and try to stay out of trouble until you can spam your spear without losing a lot of mana.
mid game: you start to do a lot of dmg, getting some kills. you can tower dive if your shield is up, by spear > stun > heartseeker > hit > run+ghost . do this only on low hp single champs tower hugging :) or running low hp people
try not to ulti into battles, you will get raped. you can ulti on running low hp solo champs, and on your own towers to protect them.
you will now be owning, destroying champs. try to stay away from more then 2 enemy champs. in team fights pick your targets wisely: casters are easy meat for you, especially with banshees