Obviously, Landis Royal Troops are supposed to be a bit better trained than this. Most troops should be able to identify an incapacitating light flash in progress and know enough to not look anywhere near it while it happens. This, incidentally, is why most mages in the armies of most kingdoms rarely try such a spell out in the field of battle; it is easy to identify and easy to avoid to the well-trained soldier. Against leser-trained people, however, it can be effective if used properly; it is a relatively simple variant of a normal magic light source and relatively easy to cast (As opposed to, say, magically locking the joints of a victim, paralyzing him or her), which can come in especially handy when rent is due.

In this situation, however, Eddie and Winslow (left to right in panels 1 and 3) simply didn't see Alex prepare the spell. They were a bit preoccupied with Matt knocking out their leader. As most people would be.