Pantheon is an excellent melee carry (yes, Pantheon is a melee carry IMO) and pusher. Your build is the typical mobafire top build and it should work well. Your early game (assuming your using the moba build, which it appears) would have your skills going something like Q,W,E,E,E,R. Holy **** that just spelt qweeer. Anyway, I find this a much more reasonable approach, as heartseeker is not crucial until you have about 200 att dammy. Q,W,E,E,Q,R, This greatly benefits your harrassment and keeps your spears hitting roughly 1/5 their hp at level 5. You should be harrassing CONSTANTLY with spear throw early game and at level 6 bp for your occult and brutalizer (if you have 2 kills you will have enough money, you should have 2 kills). From bank you should be ulting into the other lane (either top or bottom) and be picking up atleast 2 easy assists. As for your ult, its a matter of reading whats going on and what they might be thinking, but casting it behind them is never a bad idea since they are ****ed wherever they go. After that you should have atleast 2 stacks (you have level 2 spear throw, that means you should be killshotting with spear and pick up 2 kills tbh). From there you push your lane no more than 60-70% of the time realisitically, your typical W,E,Q,Q,Q,Q,Q,W,Q etc combo is now a ganking rape machine, especially since you have a good 40 attack damage bonus atleast.
The mobafire item build is kind of ****ed because if you have 3 thirsters and occult stacks, you will be melting faces but if you die your screwed. Heres my rule of thumb: If you are doing really well early game (10ish stacks on occult at level 8-9-10) and noone on the other team is giving you real trouble, buy a thornmail instead of your first thirster. This will help you so much you wont even believe. Your stacks will easily reach 20 then you can work on your thirsters. Alternatively, If your doing kind of ****ty, get your first thirster then go for thornmail (or guardian angel if its appropriate at the time). There are magnitudes i could say for pantheon buy goodluck with this.
P.S: Masteries: flash/clarity, teleport/clarity. These are pretty much nonnegotiable.
P.P.S: Using your ult behind an enemy turret completely in the fog of war when they are low health and bping is a ****ing GREAT idea and is pretty much the most satisfying thing in the game.
Source: 100+ games with pantheon