In the early game you wanna harass your opponent as much as you can. if your against a squishy champion and you think you can kill him then start poking at lvl 1 if not i recommend to wait when you have 2 points in your Q en then start harass normally you scare them off so you can farm and deny their farm if they come close just repeat normally your opponent is greedy for farm and ends up dying if not just deny them and wait for mid game.

You are probally fed right now if not push your lane and look for a opportunity to kill their mid laner. if you are fed try to kill them or harass them out of lane if they leave go for tower but remember to ward properly so they cant 1 vs 2 you. if you get it help other lanes to win their lane also and go for objectives if your opponent is a split pusher ask your jungler to defend your tower sometimes and always push your lane!

This is where pantheon falls off in damage but you can be usefull the enemy team probaly groups right so you need to teamfight you probaly have a lead because you and your team took a lot of objectives in the mid game. Try to tell your teammates to engage on their team and ult in the center of it. they probaly think is a 4 vs 5 so they react on it a ulty can deal good damage on a adc so when your land on the ground try to inmidatly stun their adc and kill him if this goes well the teamfight should be won and try to end the game.