Pantheon is a great assassin but requires skill to play. With the right build (dependent on other teams composition) makes him tank and counter the other team champs effectivly.

Pantheon does great on Armor Penetration rather than AD. Ive experimented with him many times to get to this conclusion. I have him starting at 39 armor and 39 M.Res, the armor levels faster than the M.Res but does not stay far behind.

Pantheon always has a high damage output when spamming long sword. If you build Black Cleaver, Half-Mallet/Half Hydra, and maybe Half-Mer Scimmy, he will tank easily. The reason why I picked Mercucial Scimmy for my def, is due to OP champs. Lissandra, Akali, Ahri. Have to much M.Damage output and make it hard for you or allies to kill them. Since many people usually dont have Magic Pen it makes it easier for you to strike against their AP whoard and tank. One thing is sure, look at your opponents items when you can, if you see Magic Pen being build on the OP champs, only get the some magic def unless they drop the items. The Frozen Mallet helps with your HP (3027 If on full build) and helps you run away when your low. The infinity's edge helps with his Passive (100% Crit Rate for Champs under 10% Health) and helps you strike a 400-500 on your Q when their under that health. One thing is sure, make sure u build two Pickaxe's after you have Cleaver,Mallet,Mercucial Scimitar.

Here I just made sure to maximize his damage output to compliment his armor pen.

I always lane Pantheon top most of time, I always max his skills in this order; Q,E,R,W. Reason for this is that his Q will be his highest damage output in lanes in the beginning, his R will be his second highest due to its AOE (Attack on Everyone) effect and helps farm heavenly. His R, can be used for multiple things, escape a gank, intercepting, turrent diving (since the guys always want stay at their turrent when they are low, easy to land it), and best of all mobility.

Pros:
Pantheon starts out tough if played right
Can counter NEARLY ANY champ including Darius, Akali, Garen, Volibear, Olaf, Tryndamere.
His shield lets him sneak a skill here and there while a opponent is hugging the turrent.
Cons:
Bad team composition can prevent Pantheon from doing an outstanding performance in Solo/Team fights
Can easily Kill Steal a ally due to his passive
Ult very hard to master when targeting and easy to dodge.
Not enough farm can ruin his early game if not getting any kills.

Pantheon may be tough as he is, but he is not immortal. He requires his teams cooperation when he is at an unfair advantage (i.e. 2 vs 1 Lane, low heatlh vs full health). But that does not mean only fight for kills, if you can save your ally while their chased by a mob, stun the champ nearest to him to drag aggro. If you see you've been K'Sting, and not really helping, suicide yourself to save another member, makes you look good when your doing bad.

Make sure you get the last hit on minions all the times you can. If you farm him well he will have a higher performance late game since he already has his items. His E at 300+ AD and r4 will let you kill all minions with it except the gunner.