It's pretty simple.
Unless you want to get complicated by using the light to apply pressure to spots while grappling (I don't like grappling if I can help it), you basically use the light as a hammer fist.
Face and other exposed bones are the target. If somebody grabs you, slam your fist, with light protruding, down on the bones that are exposed in their hand. Immediately bring your hand up and strike the face. If you're positioned where it's easier to strike the face, do that first, and then go for the hand if they've forgotten to let go (people often tend to "clench" under stress, even when it's the wrong move).
Think about what you carry when you can't have a weapon. I think knives are tools, but, let's assume that you're someplace where a knife will also automatically be considered a weapon. What can you have?
Water bottles.
Small flashlights.
Keys, as long as they don't look outrageous. (Kubatons may not be allowed.)
Magazines.
Of these, a large, heavy magazine would be my first choice for most defensive applications. Roll it tightly. It can be used to strike the hand or wrist of an attacker, or their face. It can also be swung, with a bowling type motion, into their solar plexus.
You may have to think outside the box. For years, I've always carried a sturdy lock on my carry-on bag on flights. Attack a lock to a belt, and you've got 3 feet of distance you can use to your advantage, especially when the attacker is armed with a plastic box cutter. I have several different lanyards made from 550 cord. These are easily explainable when you have a carabiner on one end, and keys on the other (quite useful, too, actually). I've never even been questioned about these in airports. I've carried them into places that didn't allow chains on wallets, even. Heh. Ever had a slingloading class? 550 (type 2/type 3 cord) is tough stuff.
There are only so many types of weapons. You have sharps. You have sticks. You have flexibles. You have projectiles. Spend a little time learning how to use each type of weapon, and then you'll never truly be disarmed.
John