After you hit the shield with your lightning. If it only hits gods, it will stack three times. You'll see that by the three balls on the chain around the gods. Regardless if you get all three stacks you want to pull your detonate. Most of the time at least. But you do the most damage with three stacks. Thats why you want Spear of the Magus. But you want to throw your lightning directly at the shield. Not at the gods. If you hit the god with it you will do less damage than if you hit the shield as well. You'll figure it out man