Panth is one of my favorite characters.
try using a combo with his abilities for fast and amazing damage.
1.Aegis of Zeonia for a quick stun
2.Heartseeker Strike for fast and amazing damage
3.Spear Shot to try and finishing them off
Exhaust xomes in handy for when your combo isn't enough, it will give you enough time for Spear shot cooldown to be ready again.

i stack mainly armor pen so you can also shred through tanks as they usually have alot of armor
Try using Grand Skyfall to cut enemy champs off when they push too far towards your turrets, try placing it right behind them so they have no choice but to run towards your turret or right into your ult. this works great for defending turrets as the turret will blast them if they do the slightest damage to you.