[Master Shake]

If you have access to astral 2 mages, Soul Slay is probably your best option. Giants rely on few, very tough units. Soul Slay kills units regardless of the nunber of HP. Northern Lights banner is expensive (40 astral gems and I think Constr 6). A Starshine Skullcap (+1 astral - requires 2 or 3 astral to build) and Crystal Coin (+1 Astral - requires s2e2 to build) are cheaper, but only affect one mage. If he desn't have mages out in the field, use the +1 astral empowerment that boosts all mages on the field. Also, stick a Medallion of Focus on.