Pantheon mainly has two main playstyles you can go. Anti-carry high damage burst build, or frontline sustained damage build.

High Damage Anti-carry
LANE: With this build, you want to look to maintain a lead on your enemy laner with ignite pressure and high damage burst. Look to push the laner under turret if enemy jungle is not pressuring you and poke them with empowered Comet Spear. If you can get them low enough, look for solo dives onto the enemy. A 1 for 1 is favorable if enemy does not have teleport or if it is down since there will be a huge wave under turret and they will miss all the XP, gold and the wave will push towards you when you come back to lane. If enemy jungle is pressuring you, look to keep the wave on your side of the lane and poke. Play with confidence, if enemy laner is low, you CAN 1v2 if you play it right.

MIDGAME AND LATEGAME: Objectives will be key, if you aren't split pushing and pressuring your laner, you are looking to find picks on the map with your Grand Starfall. If you can find a pick you can 4v5 objectives. If there is a fed ADC or Mid laner on the enemy team, you are the Anti-Carry. There is little counterplay at 3+ item Pantheon if you flash ignite Shield Vault combo the enemy carry. If flash is down, look for flank or Grand Starfall angles to get onto the enemy carry and misposition enemy team, allowing your team to front to back safely.

Frontline Sustained Damage

LANE: Lane plays out similarly to high damage anti-carry build. The same fundamentals apply with wave management and pressuring enemy laner. One key difference is that you are taking teleport with this build. You are able to Grand Starfall to midlane or botlane, get a kill or help your team, and instantly teleport back to top to maintain pressure so the enemy laner doesn't get back in the game through tower plates or split pushing.

MIDGAME AND LATEGAME: This is where the main differences in the different playstyles appear. With Goredrinker, Spear of Shojin, and Overgrowth, you have insane hp and sustained damage. In midgame and lategame you are looking to punish enemy mispositions and provide peel for your high damage carries. You can engage and combo on someone who mispositions. With your goredrinker and high hp, you can safely disengage with Aegis Assault if the fight is unfavorable and try again when you find another opportunity.

This goes for both playstyles, do not, under ANY circumstances, try and full combo someone with Bone Plating active. Proc the rune first before you engage.

Conqueror is the primary keystone for the sustained damage build. You stack it incredibly quickly and are able to sustain off of it aswell with long trades and fights.

Press the Attack is the primary keystone for the high damage build as it provides insane burst damage with your empowered W.

Triumph used to be good, but after nerfs it is inferior to your other option.

Presence of Mind provides great mana sustain in lane if you reliably poke with your Comet Spear. It also allows you to take Transcendence instead of Manaflow Band with its mana sustain.

Legend: Tenacity is the only reliable option in its section.

Last Stand is also the only reliable option in its section.

Transcendence gives scaling ability haste and lets you spam more abilities.

Scorch is very useful in lane, letting you get extra damage with your [*] Comet Spear poke.

Bone Plating is easily the best rune in the resolve tree. It prevents damage in short trades and all-ins.

Overgrowth allows you to have more hp, especially with the frontline sustained damage build.

Always go double adaptive force and respective damage reduction type.

Core Items

For high damage anti-carry, your core items are:

Eclipse will give you a shield and high damage burst at 1 item.

Black Cleaver gives 30% armor shred, along with your ultimate passive, you will cut through tanks like butter.

Blade of the Ruined King insane burst damage with your empowered [*] Shield Vault

For your frontline sustained damage build, your core items are:

Goredrinker gives you hp, damage, omnivamp, and ability haste with an active that heals you.

Spear of Shojin gives insane health, damage, and ability haste. Its passive also synergizes well with your basic abilities, giving a lot more uptime for your abilities.

Optional Items

Serylda's Grudge is one of the best items regardless of what build you are going. It has slow, armor pen, ability haste, and insane damage.

Death's Dance is great for the sustained frontline build, allowing you to take damage over time allows you time to get a kill and heal from the damage you took.

Maw of Malmortius is great for a single AP damage carry, but you might be able to find better use of:

Anathema's Chains is really one of, if not, the most underrated items in the entire game. It can completely nullify a single carry giving you a lot of issues.

Force of Nature is a good item if you are facing multiple AP carries with the frontline build.

Guardian Angel is good for either build if you are facing long team fights and need to get damage out fast and feel you will get bursted too fast if you go in.

Sterak's Gage is good for the frontline build giving you a lot more hp, and a huge shield, allowing you to frontline reliably.

Serpent's Fang is a great item for the high damage build if you are facing against enchanters that are preventing you from assassinating a carry, or if your laner has large shields built into their kit.

Edge of Night is another good item for the high damage build that prevents you from getting caught out by a single spell.

Chempunk Chainsword is useful if you are struggling to win fights against champions that have significant healing.

If you have any questions regarding more specific examples or situations; add my discord and I will be more than happy to help. Discord: xskyswitch

I am an Emerald top lane main and this guide was made off of my experiences with these matchups as well as studying and watching higher elo Pantheon players.