For a person to be a troll slayer online, one must have some very uncommon traits and be in a situation in which the troll cannot find weakness. Of course, we are talking specifically about the nasty troll who wants to bully and abuse simply to elevate their own deflated ego.
The first requirement is no family. Those with their own children should not try to slay trolls, leave it to us who do not have such a target for them. The nasty troll will go after family, even very young children, in ways you cannot foresee or defend against. Protect your kids by not engaging the troll at all.
You must have a career that is immune to rumors. This one is often overlooked until it is too late. If your company is picky about your online life, you should change companies, and not engage a troll until you have done so. No matter how well you avoid the connection, the nasty troll will always find it and make your life worse.
Political ties are fuel for the troll. No matter your political ideology, you must be able to laugh about it. The nasty troll will use this as a weak point if you take your political views too seriously. If you hide your political views they will merely make it up anyway.
You must laugh at all your flaws, and also be willing to make some flaws up. Preempt their mockery by cracking jokes using topics most people would be insulted by. Cracking jokes about how fat your mother is often confuses the nasty troll, it steals their thunder. When they use an insult just use the smile and nod technique, adding in a humorous bit to your reply will destroy their insult completely. Your skin must be made of steel.
Patience is important, remember, the nasty troll is persistent. It may go on for days, but while they are distracted with you they will leave others alone, and that is what makes you a troll slayer. Their brains are not complex or large enough to handle more than one target at a time. You include also incorporate some of your allies, which will make the nasty troll slip up a lot.
As you can see, being a troll slayer is more about the circumstances of your life than about skill. If everyone who can safely engage a troll does so in such a way, they will eventually go extinct, unable to get the reactions they want and always being drawn away from targets they can actually hurt will reduce the damage they do, and frustrate them to a point of exasperation.