Pantheon is often seen as a glass cannon AD nuker. While this works well in lower end games the problems with being a melee glass cannon will become very obvious later on. Stacking HP and getting Atma's Impaler you will cover AD and survivability at the same time.

This section is if you use this build in SR.

Mastery Tree.

The masteries go heavy into offence:
Archaic Knowledge will greatly help with your Ult damage.
Lethality helps with Pantheon's passive.

The 9th Point in the support tree can go into Greed , Meditation , or Utility Mastery . It is a personal preference.

Summoner Spells

Flash: This spell is 100% required. If you do not have this spell you're instantly at a dissadvantage from every other player in League of Legends. If you suck at using Flash, invest the proper time in learning it well.

Exhaust: This spell is more flexible, it can be changed out for whatever you prefer, but do not underestimate it. Exhaust can easily change a game or a team fight your team may have otherwise lost.

Runes:

Greater Mark of Desolation: Should be obvious enough. It's a top tier rune and pantheon is all AD.

Greater Seal of Replenishment& Greater Glyph of Replenishment: are more of a reassurance in case you're required to play very defensive and rely on Spear Shot to last hit.

Greater Quintessence of Health: Can be swapped for anything you would rather prefer.

Skill Selection:

You have to decide while playing ether to max Spear Shot or Heartseeker Strike first. If you're unable to get in for the stun and strike you'll be better off maxing spear shot.