Well even without the lock-on feature, Koi should be able to support the others quite well.
It's just harder for him to kill an alien. The ones that aren't engaging the Gantzers at that moment are not that hard to hit. They shouldn't be moving fast and they don't know that someone over a hundred meters away is shooting at them (at least not until the first shot hits).
And the ones that are engaging Gantzers should still be distracted by sudden explosions around them, even if he doesn't hit them.
Good thing he took Sean with him though. Because he'll probably get to see an alien up close.
Having to fight an alien in close combat without a suit is the fun of not having a suit after all.
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"