Pantheon is the kind of champion whom can cut through carries and tanks alike, his stun along with HSS combo is ideal for tearing through any and everything. I will cover how, what, and when to use his abilities and his item build.

Runes are sort of standard, Flat HP quints for the early game survivability (this combined with the aegis passive makes you very tough with 2 hp potions). Along with Marks of armor pen. because this will cut through low level armor, meaning your low level spear shot and HSS will do maximum damage. I like to grab Seals for magic resis. because the middle lane (which is where you should probably be) almost always has a hero with magical damage, or a carry. If you know for certain you are facing a carry mid whom is physical damage, then you can swap these for armor seals. Glyphs of Might for early-mid game bonus damage, these along with your armor pen. can tear through squishy carries and even late game can cut down tanks with high armor pen.

Pantheon is very strong laning partner, but he can also hold mid solo very well. Pantheon has a global ultimate, this means you can stay in your lane for as long as possible before you gank another lane, optimizing exp and gold. Skill your spear shot at level 1, DO NOT skill HSS first, I can't tell you how often I've seen a pantheon skill HSS first and hit no one with it, at level 1 it can't kill creep waves either, yes it gives you an automatic crit for enemies below 15% hp... but if you are a decent player you can last hit/spear shot creeps very easily. Once you have around 220 or so AD your HSS will start crippling creep waves, NOW you can use it, don't waste mana early on by spamming this ability.

The spear shot at level 1 is critical, it allows maximun farming and a decent "poke" for the enemy champion in your lane. I usually get my stun, Aegis of Zeonia by level 3 early, because the stun increases per rank in the skill. This means that you can optimize damage on your HSS by landing more hits on the target, whilst they are stunned longer. I like to grab early stun skill first, then maxing HSS first, then Stun, Ult whenever its available, then spear shot. Spear Shot is best used last because you are using it for poking and farming, it isn't your biggest damaging skill, until you have 300ish AD it deals massive damage with high AP against squishies.

I always grab a meki pendant and 2 hp pots, these and your passive will keep you at full hp with mana to destroy any lane enemy. I get meki pendant for the Manamune item, which gives extra mana (which pantheon needs desperately) and mana % to AD which is around 35-45 dmg when you make the item, this depends on how often you used skills with the "tear drop" item. People usually rage at me early game for getting the meki pendant, but then shortly after they shut up and thank me, because i just ulted their lane and stunned the carry gaining us 2 kills. After you get Berserker Greaves you can get manamune at the same time. Now focus on hp/AD, i usually get a Phage quickly and then get a Youmuu's Ghostblade asap. After i finish it I finish the Frozen Mallet. This combo tears through carries and beefy tanks. After Youmuu's get an Infinity Edge for the bonus crit damage that you hit everytime the enemy is below 15% hp. After you get infin. grab a Bloodthirster which will give you enough AD and AP (combined with Youmuu's Ghostblade Ap) combined to destroy any champion.