Yeah, Core Tap is pretty powerful, no doubt. It was a little strange that they changed it to work on players.
But there's many ways to counter warding/fire spells, from shield mind to raw TD. In the case of KS, they could have their magic dux + fire resist, as well as their full plate and crit padding innately reducing the damage even more. Plus you have CC breaks, etc etc..
Gear for sure plays a role, but not as much as being prepared. For me, gear is usually not my first layer of defense. But I tend to like to have 6-10 layers of defense against any given attack, where possible. Implode is really the only exception, in which case I double down on my primary defense against it. If you don't have the gear, you just double down on your primary, secondary, etc., layers of defense, just the same. As for offense, I mean, it helps too but usually it's more about the skills/spells/attacks you use and not so much the gear. But obviously gear can be significant, even more so if you specifically get your gear with PvP in mind.