Pantheon is a very good asasin top laner/jungler and he works fine in the mid lane especially against certain champions where his passive can really shine through such as Twisted Fate. He does extreme damage and most mid laners cant handle it as a majority of them are super squishy.

Longsword and Refill Pots are your best bet with Panth Mid. Longsword will let you get a Dirk faster and refills are necesary to abuse your OP early game and delay your innevitable fall off for as long as possible.

Start your Q for early harass and take advantage of it as much as possible but try not to dip below 50% mana. When you hit level 2 upgrade your W and look for an all-in. Most people won't expect the amount of damage you can do with W,Q,AA and electrocute proc. A lot of the times it will get a flash. You could also wait for level 3 to possible kill your laner, but at this point they will also be stronger so it is a big more risky.

Once you hit level 6 you should be looking to roam whenever you have your ult. You need to make all your lanes snowball so they can carry you late if the game lasts. Bot is usually the best place to go as you melt squishy champions the easiest. If the top laner is something like teemo that may also be an easy kill.

Duskblade, Youmuu's and Black Cleaver are core for this build, You need youmuu's especially to help chase down enemy champions. Items such as righteous glory also work exceptionally well with this build as do randuins and even icy gauntlet with your W then AA.

If you can force team fights early on, then you can shred teams easily with help from your support. You will want to R into the enemy team as your team goes in on them, this will do alot of AOE damage and get you into position to jump on their carry and do your W,Q,E AA combo.

Sad days, but unfortunately, though Pantheon is blessed with an amazing early, he is cursed with a terrible late. You can still be useful however as later on you will want to start building bruiser and get items such as Steraks, Thornmail or Adaptive Helm.

In the late game you want to provide as much peel as possible for your carry, keep the enemy divers/assasins off the carry with your W. At this point you should be like a second support in team fights.

Pantheon can still make good picks in the late game. He still does decent enough damage to kill the enemy carry alone in the jungle, however if there is any more than one or they're not squishy you shouldn't go in on it alone.

Pros-
Good Early Damage
High Lane Pressure
Amazing Roaming
Unexpected Pick
Cons-
Can be kited
Outscaled by everything
Outdamaged by certain asassins.