Here is my way about playing Pantheon. I never liked the way about a caster Pantheon, so I go as DPS + casting spells.

After playing pantheon more than 400+ games, I think I can give you a guide

As you see the first build is more offensive, and the second one more defesively

Offensive

ArPen Marks - This helps you as a DPS and as a caster. The most usefull for Pantheon
Mana reg/lever Seals - This make me no need manamune or clarity.
Mana reg/lever Glyphs - The same with seals.
ArPen Quintsn - Same as marks, nothing suits better to Pantheon

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Defensive

ArPen Marks - This helps you as a DPS and as a caster. The most usefull for Pantheon
Mana reg/lever Seals - This make me no need manamune or clarity.

ArPen Quintsn - Same as marks, nothing suits better to Pantheon

Just as you see. Late game you don't need the boots as double Phantom gives you 410 movement speed.

If you own them buy sword of the occult. So late game 20 stacked gives you 15% movement speed, like a 2nd Phantom dancer.

If you cant keep it at 20 stacks prefer a 2nd Phantom.

Late game just buy all the elixirs.

It's really simple. Take mid (unless there is a champion like LeBlanc in your team who is mid champ) or solo top.

Farm is easy with passive of Heartseeker Strike, just last hit. Also Spear Shot can help you agaist a range carry.

Playing Ranked prefer the 2nd build. It's a little bit defensively and not risky. Prefer solo top and farm cointiniously.

If you have troubles about low health just buy a Phage -> Frozen Mallet

I cant say anything more. My English are not good :S

Also I dont know nhow to add icons xD sry. :)

This build make me had 2 or 3 times quadra kill with Pantheon :)