Pantheon is Overall a great Champ due to his crowd control stun and his powerful spear shot. Pantheon is especially strong early game because his spear can take out a lot of damage getting the first blood for your team almost instantly. Pantheon is said to be OP due to his block every four hits, and if you know how to build him right then u can 4 hit/crit/block and ultimately solo a lot of champs.

Pantheon has the potential to be a wide ranged character. If he needs to be he can go tanky and still deal a good about of damage with his ability set and help the team in team fights as long as you aren't targeted first.
Warmogs is a good alternative for extra health rather then going for the frozen mallet late game. If its possible, get it after your blood thirster so you can hold up in a fight. if not and your team is feeding then go get it asap.
there may be a need for mercury treads if ap characters keep hammering you but I think the Attack speed the berserker's greaves gives you is essential.
may need to get a Force of nature if it becomes a problem.

So as you know Pantheon has pretty decent Cool downs for his abilities, especially his spear shot.
I start out by Harassing other champs with my spear shot and then when the moment is right start off the kill with a Spear shot -> Aegis of Zeonia -> Heart Seeker -> Spear shot (cooldown should be good for double spear shot).
this depending on the champ u are attacking garentees them to at least half health depending on if your attacking someone without life steal.
If your gonna Ult make sure the enemy has no choice but to go into it like going into a giant team fight where your bound to hit someone going straight into your Aegis of Zeonia. Or if your chasing someone, give up and launch behind them so they have to attack you.

You will have to take Risks with Pantheon. His block will do the rest.

I use Flash and Ignite but you can switch that to ghost and exhaust if necessary for team spell balance.