The spell Iron Bane works well against giants. Once the giant's armor is weakened, they die quickly against swarming smaller units. Of course this weakens your own unit's armor as well, but it is not like they are going to survive a hit from a giant anyway. The best part about this spell is that it only requires an an E1 caster with Earth boots. So if you can find an independent with E1, and then trade for earth boots, you are set.
Darkness + undead/demons also works well, since your undead hordes will hit almost every time, while the giants will have pathetic attack values (and won't be able to lower your defense through multiple attacks against a single target).
For best results, combine these two tactics against an enormous army of sacred giants that Nieflheim has spent 30 turns building up.
