Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Start with a Meki Pendant and 2 Health Potions. Next up is Chalice of Harmony and Boots. Rush a B.F. Sword after that and a Last Whisper. Complete your Boots of Swiftness and get another B.F. Sword. You can sell your Chalice now if you want. Buy a Banshee's Veil if needed. Then yet another B.F. Sword.
Ignite to make sure you kill someone (can swap with Exhaust). Flash because of obviously overpowered summoner spell and your only escape.
Farm, farm and farm. Your ulti is a great ganking tool, always use it. Sometimes go ulti a huge creep wave.
Try not to be too aggressive. Always focus the squishies. Always punish their squishy idiot standing too in front of everyone by leaping him and nuking while your teammates also come to nuke him.
Try to steal all the possible kills. Panth is very item dependant and huge amounts of golds in a short time can mean game over for the enemy team.
Don't forget, when your enemies are all clashed togehter, it's always worth it to leap someone and get all ~three of them in Heartseeker Strike, even if you die. That almost means triple kill or your teammates finishing them off fast because they're so low.
Don't think you can have very few deaths. Panth is a big nuke risky hero with no escape. Dying after damaging the whole team really good is totally worth it. Just like Karthus. Of course dying like an idiot after doing nothing is stupid.
That sums it up, good luck, and don't forget you also have a team, work with them or lose rambo.