I started playing one of the Metro games, and one of the common enemies is an armor plated Spider the size of a small dog that your character's flashlight causes its armor to boil as if the light is acid. Eventually I took a break from this, walked past the TV and the movie Pitch Black was starting. It is set on a desert world inhabited by these bug like creatures larger than humans, and have the same trait of being physically injured by light. In both instances, too much exposure to light seems to be fatal to these creatures, with the Metro case being about 10-20 seconds being enough to kill them.
Having coincidentally seeing both of these instances in such a short time span I got to thinking. How realistic is the general idea of a creature which when exposed to light of even a flashlight causes its skin to burn as if it's on fire, leading to the creature's death in a short time span? If not a flashlight, then more powerful or special sources are still acceptable, such as maybe UV lights.
It would be preferable if a human were exposed to the same conditions that would be near certain death to the creature, the human would be mostly unscathed, but the line is drawn so long as a human is at least more likely to survive the said exposure than not. The ability to weaponize the light is of a high priority here, so we don't want to turn on a special flashlight and have it effectively become a suicide bomb unless that is the only way for this creature to exist.