Not Updated For Current Season
Alright I have seen a lot of pantheon guides and most of them forgot one fact. In team fights for 5v5 pantheon is as squishy as a mage without any health items. Also manumane is a very helpful item, not only do you never run out of mana you also gain more attack damage as the game progresses. Also i have found ignite to be more useful than exhaust because it adds damage and reduces healing allowing for a quicker nuke.
As for the phases and how you should play:
Beginning game: Alright buy the Meki Pendant and head out to a lane. With this build you can easily go to mid and get a kill there so feel free to go mid get your ult early and its open ganking season. For gameplay harass them with spear shot and as you lvl up get your stun and heartseeker strike. If you have not already killed an openent do so at lvl 5 your heartseeker strike should be able to kill almost all champs if used with your ignite and spear shot. (this is provided that you harass them with spear shot. After this recall and buy items and use your ult to gank if you have not already used it.
Middle Game: Mid game is where panth is at his best when u get the chance use your ult to gank and stun + heartseeker strike should kill most enemies. Get the items i recommended and if also the chance arises use your ult to back door turrets.
End Game: By now you should have FM making it hard for most enemies to escape as well as make you incredibly hard to kill from this point its gank/ recall/ pus/gank and a gg.