Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Focus on damage as long as survivability isn't needed. Look for situations where you can skyfall, if the enemy has little health, landing on them at the tower is almost guaranteed a kill, try to position it so that if they run, they opt to run towards your allies, and not back towards the next tower.
You can backdoor, but since attack speed is not worth getting on pantheon (only attack damage affects your skills.), be certain the enemies won't get to you in time, you'll hit hard but slow.
Pantheon should also grab elixir of fortitude every now and then when money allows.