The skill-shrouded heart had been used, one that consumed all of the user's mana, regardless of their stage or the amount of mana they possessed. All of it flowed to one place, to safeguard the mana core surrounded by the heart.
It wasn't a final defensive move, per se, because the skill primarily protected a single location. If Feebie had chosen to launch a final attack towards his head or elsewhere on his body, he would still be injured, which explained the large wound on his back from the attack.
In most cases, it was easier for the target to move the head. The skull was also a good defensive measure and Raze had learned that in martial arts, in general the techniques learned weren't attack's aimed at the head.
Whether this was some type of honour code he really wasn't sure, but it meant with probability that Feebie was most likely going to aim for the heart if she wanted to kill him.