Pantheon is a great all in champ especially on single targets and is also a great roaming champ as a mid lanner he can easily help bot, top, or jungle with his ult. For the most part you want to focus on CS at the beginning of the game at least until you get level 2 or level 3. At that point you have all you need to all in the enemy lanner as long as the enemy jungle isn't around. An ally gank is always helpful too. To start your combo when the above is met is to attack minions till you get his 5 passive stacks which allows his auto attacks and Q to do more damage. Next get close enough to the enemy lanner where you can land your W on them try to be inconspicuous but they will likely see it coming. Next use your W followed by your 3 auto attacks this is your primary damage source. Next either Q or E depending on health and ignite if you absolutely need to, however NEVER NEVER NEVER flash for a kill at the beginning of the game. This is mainly because you never know if you are going to get ganked by the enemy jungle and you need it to escape.