Are you trolling? Seriously the only AP carry that belongs in the jungle is fiddle, karthus is a gutter tier jungler, and the rest of the AP carries can't even jungle without seriously screwing up their build orders, and rushing a hexdrinker every game isn't ideal, as it sets your build out of order. Also, AP carries have significantly better burst than pantheon, take annie for example, without any AP items, her burst is about 1000 damage, and it happens almost instantly, whereas panth will only burst for around 500 damage in the same amount of time.

Now I will use numbers to explain why infinity edge is NOT necessary on pantheon:
Your average AD carry (who you should be killing) has around 2000 health. 15% of that health is 300. Without an infinity edge, just one spear toss will deal around 360 damage when they are above 15% health, but pantheon's passive is overkill, when taking into account that the spear toss would kill them WITHOUT the passive in one hit if they are below 15% health. Now take into account pantheon's passive, when they are less than 300 health, your spear will deal about 720 damage, so they WILL die. Now if you add an infinity edge, your spear will deal 900 damage instead of 720 damage, but that extra damage goes to waste because you only needed 300 damage to kill them in the first place if they were at 15% health. Now if you want to do a bit of math, an infinity edge will only be necessary to kill someone with 4500+ total health. (Say your spear toss without infinity edge deals 700 damage to a target at 15% health. Let x = total health, .15x = 700, 15% of some total health is equal to 700, x = 4666.67)

Also, you obviously didn't read my guide, because in THE VERY FIRST SECTION, I point out that the cheat sheet's numbers ARE WRONG. It doesn't take into account the bonus AD you receive from Atma's impaler, which is around 65 AD, setting him at about 240 AD, which is plenty of AD for someone that also boasts 240 armor and 3.5k HP.

And his ult definitely isn't made for the damage, if it was, AP pantheon would be as popular as AP shaco. If you use it for damage when you're already in a fight, YOU WILL LOOSE DAMAGE PER SECOND. You take 3.5 seconds to deal about 1000 damage before resistances, IF YOU LAND RIGHT ON TOP OF THEM, but more often than not, you'll hit them close to the edge, leaving you with about 600 damage, without any magic penetration, that will be reduced to about 400 damage at level 16. One spear toss will deal about 360 damage before resistances and about 300 after resistances (because you should have armor penetration), but without a ridiculous 3.5 second charge time, and your spear toss isn't even your heaviest hitting ability.

I never said pantheon was a tank, but if you want to be effective, you need to build him tanky, or else you won't be able to survive long enough to kill the enemy's AD carry. Also the magic resist he has comes from mercury's treads, which aren't built for the MR, they're built for the tenacity.

First of all Hello :).
Here is my feedback/suggestions.
Runes:
-You should really go for full Arp Marks-Quints.Strength Runes tend to fall of quickly,thus making your Spear shot and heartseeker strike doing silly damage after a couple of levels.
-Greater Glyph of Clarity?.Its really not needed.Especially with the items you are using(Sheen,Glacial Shroud-Frozen Heart).Try using Magic rez/lvl.It might suit you better :).
Masteries:
-Although going full defence has some advantages,you lose extreme amounts of dmg if you dont go deeper into the offensive tree.
Items:
-Unbalanced Stats.Kogmaw will eat you alive,and Ap carries will be dancing above your flaming corpse.
-Weard item order.You buy Sheen,Glacial Shroud,Giants Belt.Almost 4k gold.Those items are nice in generall but they dont add anything drastic to your character,witch will probably end up with you getting ignored by the oposite team,and then die.
Try using Banshees Veil instead of Frozen Heart,and grab it against heavy Ad if needed.
Although its nice that you start with Dorans Blade.Really works well.
I dont know these are my thoughs.
Gl & Hf :).

Ok so as to where to put your ap carry you put them in jungle (some can not gona go into that right now but for examples mainly fiddlesticks or karthus). 2ndly you can out burst ap mids, your weak from lvl 1-2 once you hit 3 and have your combo you can burst any standard ap mid down if not kill (exceptions are leblanc cause shes just straight up annoying and you may also have trouble with kayle if she goes mid). Also if you are really having trouble with ap burst then just rush hexdinker because that will save you from almost any kind of early/mid game burst. 3rd infinity is not overrated do too the increase in crit damage which you have guarenteed when ever they are below 15% hp. This is the main source of your damage along with maybe ghostblade, trinity is not needed though sheen built into trinity late game can be very viable depending on game situation. Also in your main build you have magic resiste. I see that you address some of items in your item list but you really only discuss items that ive NEVER thought about using and you say not to use them so im not sure what thats about, if you want me to list my main items i will. Finally somthing i didnt catch the first time i read your guide was your use of his ult. His ult is not mainly for teleporting, though that is usefull at times, the main uses is actully damage by droppoing in on teamfights or but jumping past a tower on someone blue pilling at low hp because your passive will block the first tower shot so this is a very very very effective clean up method.

To summarize my main issue lets look at numbers, you have 3.6k hp and only 172 damage? you scale off damage so this only hurst your burst (as you said he is a ad caster not auto attacker but your giving him no burst as he scales of ad) and 3.6k is excessive your NOT a tank. A good target range is 2.5-2.8k (if you need surviablility) and get that damage above 250 at least.

Frozen mallet and infinity edge are both overrated items for pantheon, they may have more raw AD, but you're squishier with them and they don't help your abilities like triforce + atmogs does.

Pantheon normally can go in many lanes, but with this expensive of a build, he NEEDS solo top so he can farm as much as he needs.

Maxing heatseeker strike is risky and pushes the lane to hard too farm consistently with it, so I feel it's wasted until later levels when you can safely push a lane to the tower and then go gank safely.

Also pantheon really doesn't stand a chance against an AP caster mid because he can't outburst them, he can't out range them, he can't outfarm them and he doesn't have anything that blocks their damage, and where do you put your AP caster if they aren't mid? AP casters scale with levels so they take mid, AD casters scale with farm so they take top.

I take a point in W first because it just has more utility, as it has a good CC and can close a gap if needed. With a point in W at level 2, the chances of a successful gank on the enemy increases drastically because CC is more important than raw damage in ganks.

The reason why so many people find pantheon to be a bad late game champ is because they all build him too squishy and still expect him to be able to go through the enemy team to kill the enemy carry. If you want to be able to do that, you need to build tanky, and there is absolutely nothing wrong with being more tanky, as in the long run, you end up doing more damage, because you stay alive longer, so you do a bit less burst, but you maintain that lesser burst for a much longer amount of time and in the end you will have done MORE damage by building less pure damage items. Trust me, once you reach higher levels of play, you will realize building squishy is sub-optimal on almost every melee champ.