Mortal Will activates after 5 spells or basic attacks - plan your way through fights to allow you to activate it more than once.
Whittle the enemy down with Comet Spears before you jump in.
If an enemy is going to get out of range of your Aegis Assault, you can recast the spell to immediately fire the shield slam.
Pantheon's Aegis Assault makes him invulnerable from the front. Get behind him or wait it out.
Be careful near Pantheon if your health is low - a thrown Comet Spear executes low health enemies.
Grand Starfall gives several seconds of warning before Pantheon arrives. Use this time to get out of the way or to form a plan to deal with him.