Build
Genius and Glowing enables you to even pick off tanky heroes like Uranus, Alice and Esme. Greatly enhanced by Divine Glaive's penetration and extra magic damage from holy crystal. You can buy winter and swap out in order (Winter > Immortality > Blood Wings)
Emblem Set
Extra Penetration, more Damage to help Gusion in finishing enemies, and Lethal Ignition for burn damage (useful in early game ganks)
Battle Spell
Able to stun enemy upon moving to your first skill which unables the enemy's dash opportunity or flicker. also great for catching heroes off guard in the bush
All phases of the game
Gusion's Passive that's based on MAX HP damage is very useful in taking down tankier opponents. His magic scaling is high so take advantage of that.
Best With Kalea
Great and easy pickoff, Predictable making it easy to land 1st skill.
Watch Out For Badang
His skill can stun lock you before you can even finish your combos.