The downside of using the anti cc preventively is that you don't get the immunity (it's the same when your resist a CC), so if he has another C of the same type or a very short cd he can CC you again right away.
So it's a risky bet, but if it can let you land a kb on a clothie it's usually a winning bet
Soldiers have enough burst for it to work. I'm not saying you have a 100% chance of killing a clothie in the time of a kb but you should take so much of his hp than with your remaining CCs and bubble you can finish him off. Demos can escape from that kind of situation though with the speedbuff (but it's not an instant cast so still not 100%).
Don't forget to go frenzy when you land a kb too. And then switch back to def stance right away unless you have bubble - also don't forget you CAN switch stances when kbed, rooted, snared. In def stance with pvp gear a necro won't kill you that fast and the dots will hurt but not faceroll you since he won't go SW in a duel. Demo can hurt you a lot in def stance though, and they have instant casts and stuff. But as I said demo is very strong in 1o1 and this game isn't a duel game anyway (so some classes simply CANNOT beat others given equal skill), so if the demo is good there is frankly nothing you can do besides being outright better/lucky.
Last, use your environment. Block line of sight and stuff, don't just chase the guy, especially if it's a demo since he will run faster than you for a good part of the fight anyway.