The giant was initially firing on both groups to its left and right, but after Ylfa's group moved, it turned to its left and marched through the train cars themselves. Quentin's attack damaged a leg, but Vaikon's shot to the head disrupted its sensors. Because of that, it's stunned and on all fours for the time being.
However, they're both exposed to the militant closest to Terrance and Rachael, who in turn is exposed to them, since it was taking cover against those two, and not the two Mindy clad marines.