Nothing like that, but you can attempt to gain inspiration through a passion check. If you gain the inspiration, and your weapon skill is high enough, you have a much higher chance to crit.
Discretion is the better part of valour. You might also go retrieve your horses, and try to lure the giant into the open for a spear charge.
He's unconscious below 17, so 22 points of damage will take him down.
He does enough damage to knock you guys around pretty bad, but he's now splitting his attack 3 ways, so his effective skill is only 6.