Well most of you talking that pantheon is easy to play , I admit this - its very easy when u know most of his tricks.
To do extreme damage with him you just do Aegis of Zeonia , Heartseeker strike , Spear shot or simply (w , e , q).
Some tricks I like to do when I see that enemys got low hp and runs to turret that thinks you wont chase him. Without youmuu's ghost blade and Aegis protection buff ready I wont prefer to do that. Anyways since enemy tries to recall you go quickly cath him do the w e q combo then quickly press yoummus blade active to escape.
Other trick I like to do with Grand skyfall when I reach lvl 6.
When you are at ur base regen spot chek the other lane if no 1 of the enemys going to brushes.If they do and your team mates are close nearby feel free to go there , eventualy you will get kill or assist. Grand skyfalling takes many mana that you will need for w e q combo so I always do my ulty from base regen spot so it wont take me any mana.

Quins 'n' Marks: Armor penetration - to do heavy dmg with combos
Seals: Health per level - to be a bit tanky
Glyphs: Cooldown - to do over and over ur combos

Exoust will help you run from enemys or keep em slower not to run from you when u gonna chase and try to do critical hit.
Ignite will help you kill enemys who thinks they are safe next to turret with low health. So when your special ability "Aegis protection" is ready to protect you run fast to the enemy ignite him and if u think u have enough Health to kill him for sure then press a spear shot skill that it wont have a chanse to survive

Well with this build I like to start with 3 health pots and boots.
U ask me why boots ? well its simple boots really helps me to chase enemys at start or run from em when they chaseing.
WHY NOT ? to start with long sword or something else. Well easy to answer while I you such a heavy dmg with armor penetration u wont need any extra damage boots are more usefull for you. Later on I improve em to ionian boots.
Youmuus blade helps me to do tricks , gives me extra damage and clean ran out.
occult will improve your AD per stack which really helps you to improve your kills in gangs or solo.
Infinite blade Alots of AD + higher crit dmg.
Last whisper will increase ur armor penatration and gives some AD.
Now CHEK: this out Armor penatr.runes + youmuss blade + last whisper + AD = serious damage to enemy
For my last item I like to get bloodthirster. Lifesteal and AD always helps