How to generate Frenzy Charges

Forbidden Flesh/Flame

Gaining the Forbidden Flame/Flesh combo for Raider adds a 10% chance to generate
Frenzy Charges on hit against a Rare/Unique enemy

Mark Mastery

The Mark Mastery has a option to add 10% to gain a Frenzy Charge when hitting a marked enemy.

Blood Rage

Blood Rage adds 25% chance to generate a Frenzy Charge on kill

The Green Dream

The Green Dream adds:
Passives granting Cold Resistance or all Elemental Resistances in Radius
also grant an equal chance to gain a Frenzy Charge on Kill
Putting it somewhere and adding cold resistance via tattoos yields a high chance.

Annoint Disciple of the Slaughter

There are better annointments but you can also annoint Disciple of the Slaughter to add an 8% Chance on kill

Legacy The Green Nightmare

Legacy The Green Nightmare had
Passives granting Cold Resistance or all Elemental Resistances in Radius also grant an equal chance to gain a Frenzy Charge on Kill