This is quite straightforward for the majority of matchups if you know how to play Pantheon even a little bit. For some extra tips, a full combo would be...

Empowered W - Q - A - Empowered E.

The empowered E gives chase potential however if they can't run away then use E instead of your second auto and wait for an empowered Q for slightly more damage.

To sum up the quide, mages are the most annoying matchups but as long as you're playin safe and looking for cheeky W's the whole time it's easy as pie. If it doesn't work out I'm sure you're the problem.

:)