Some of the enemies in the game can turn invisible. To see them use (-). You will see a stream of them moving. So now you can hit them. And it is easier to kill them when you use combinations of (B) and (Z).

When you are fighting a boss, if you defeat them in either under 3 minutes or 5 minutes you will get an XP bonus

Hurt Giant a little. Then use your feral lunge on him. You will tear of his headgear. The hurt him until he is about to recharge his health. The jump in the air and hold Z. You will pound the ground. While Giant is on the ground, beat the crap out of him until he dies.