This is an awesome card, but I have a question about the Mana requirement.

This card costs 5 mana to summon, the card text says to put 2 cards from my mana zone into my graveyard when this creather is put into the battlezone. Do I really need 7 mana to summon him, or can I use 2 of the 5 mana cost ?

You can sacrifice any 2 mana cards, including the ones you tapped to play him.