Don't waste the sp on Ground Charge. As Pawn said, Earth Shock is much better - you can manage with the other skills until you can get it.
As for Fire Blow, I have it and use it only on Party Giants, sometimes Party Champs. You could use it on regular giants if you're soloing. Don't use it on regular mobs. It slows down your kill rate because of how long the skill lasts and it will also do damage and aggro any mob between you and your target. Be ready to target another mob with a different skill if you accidentally aggro something. The total damage is great when you are in zerk.
