As to dwarf blocks, its a general consensus that you should not go over 25-30 per block. Any bigger and it becomes terribly unwieldy, and makes it so very easy to flank. A personal preference is 25 (five wide, five deep) or 20.
Royal_Marine_Machine has a good suggestion for the giant (and of course you're allowed to answer!). Otherwise, a lucky shot with the GT would work. But the BT would be ideal - D3 wounds and +1 to hit due to a large target. Add in the Master Engineer, you'll be hitting on 3s or 2s. Two or three turns would see it off. Or, two or three BTs.