[Shovah32]

As said raiding will help. Combine that with a large number of battle-axe abysian infantry and as many fire mages as possible(if you can research up to pillar of fire that could be a big help) to burn his giants to death. High strength battleaxes are great for damaging the giants a bit and massed fire spells(falling fires and pillar of fire preferably) can do a large amount of damage to them. Another option would be taking a fire9, high dom, high prod pretender and using burning ones who, while not being very resistant to giants attacks can do great damage with high attack, high strength, flaming morning stars.