I dont like that "ad-caster" assasin pantheon builds as no matter how much damage and penetration you get, you will get troubles to burst down your enemies late game, when theyll get some fat and defence, but on contrary they wont have such problems with, as you will be squishy and have no hp and defence. So I tried to build panth as early game ganker with quite high dmg who turns into tanky dps initiator with quite nice utility late game

Aspd marks (to improve jungling speed and make his passive proc more often)
armor seals (as we have jungler/tanky dps)
any glyphs you want (I prefer mresist/per lvl)
armpen quints (for jungling and higher early dmg)

17-4-9 for faster jungling early dmg and some speed. If you are not jungling you can also try top lane, where 0-21-9 looks better)

armor+ 5 pots for first clear;
mardred razors as he is ad jungler;
boots;
phage for some hp, slow and dmg;
mercury threads or other boots you like (ninja tabi vs ad heavy team, berserks if you are like 5-0 at 6 minute);
hexdrinker- its good when you gank mid lane ap casters who are several lvls higher then you, its shield will save you if something will go wrong, and usefull later in teamfights;
randuin omen parts:
HoG for some hp, chain vest/wardens mail- your main armor;
frozen mallet (major hp item with some ad and nice passive)
randuins omen (nicely landed ultiamate and then randuins active will significantly slow enemies movement and atack speed);
Maw of malmoutis- obvios improvement for hexdrinker, which will give you quite nice dmg if you will end up focused (and it suits you because you wont die that easy even with low hp)
mardred bloodrazors- for late game, where you wont deal nice dmg with your skills, enemies will get tonns of hp, and where you possibly be forced to backdoor with your ultimate, aspd and some dmg from bloodrazors allow you drop towers and inghibitors fast;
executioner calling- situational item vs team with mundo, ww, soraka, etc, which give you quite nice sustain too
Other situational items are usuall- banshee, QSS, thornmail etc

start from spear shot as it cheap and make ur passive work more often, it will let you kill blue buff, red buff and golems without losing lots of hp. Then max it as fast as possible;
aegis of zeonia- get at lvl 2 for early gank after blue, it will help you against counterjunglers (stun and instant full stack of passive);
heartseeker strike get on lvl 3 for passive mostly. You wont get great ad with this build, so active wont be your main dmg source, so max it after spearshot;
Ultimate lvled as usuall- 6, 11, 16

flash and smite for spells as you are jungler and flash is no brain for most champions. You can also get top lane with preety much same build, get ignite instead of smite

Start with blue buff, ask for pull, combine spearshot and autoatacks to make your passive work. Ask nearby lanes be prepeared for early gank right after blue buff.
Pantheon farm faster and safer camps with fat monsters (golems, lizard, ancint golem), as they just hit in his block alot but he farms wolfes and wraiths quite slow, so try ganking alot instead of farming, or at least steal enemy buffs and golems.

try drop your ultimate right after teamfights starts, so enemies will be forced to reatreat or being split if they winning fight or just die with no escape if your team more succesfull. In late game try stun their strongest carry, get focused to use your randuins passive, then use randuins active, flash out of fight if forced and then prepare to land ultimate.