Most Giants carry a big axe or a club, often a bit of tree trunk or even a big bone. If they have no weapon, they simply use their enormous fists. The Giant brings down his club with a single mighty stroke. The target may be any model in base contact with the Giant, or any model adjacent to such a model - Giants have quite a long reach so they can choose their target.
The target may attempt to avoid the blow by rolling equal to or less than its Initiative value on a D6. If avoided there is no effect, otherwise the club causes D6 wounds with no armour saving throw allowed. On the roll of 6 the Giant inflicts 6 wounds and his blow is so powerful the club sticks in the ground and he must spend his following hand-to-hand combat phase pulling it free.