Since the spear weapon's targeting system keeps coming up, every time we see it used we always see the projectile directly forward in its initial thrust of movement, before the targeting system actually kicks in and takes over to redirect. When Feral is firing without his mask on, he's still aiming at his targets even though he isn't actually locking onto them with the mask. Naru planted herself just a bit too far out of range for the projectile to hit her before the targeting system kicked in and redirected it. Had she been a foot closer where Feral was planted in the quicksand, she would have been dead.
Nothing about the way Feral uses the weapon, in the final fight there or in any prior scene, makes him seem incompetent to me, in regards to understanding how his tech works. Naru kept herself out of range, and Feral misjudged his shot/the distance (and hey, we know from the original film that without the mask on, a Predator's vision isn't all that hot).
