This Gigant is locked up in the Underground Prison inside Riville Tower. You must accept the sub-quest Survey Says from Brogen to get the key for the cell.
Head over there and unlock the cell to fight the Gigant Zeugle. This enemy is strong but you can match its power by consuming a food to boost your attack at a campsite first. The Gigant’s core is on it’s front legs. As Alphen, consider using Reigning Slash as your main offensive arte – it is slow but the impressive damage output more than makes up for it.
Lunging and pouncing is pretty much all the enemy does. But it can do them with little warning at times so try to avoid attacking from the front with either Alphen or Law.
Polycephus also becomes really dangerous when its low on health and enters overlimit. At this point, you might want too fall back for a while and wait until the beast calms down.