Playing a demo is about positioning and using your team. If you overextend and get greedy you die a lot.
If you sit in the extreme back line and SS all game you might steal some kills but the end result is your not really doing anything.
You need to find a balance between supporting your team and supporting yourself with your team, which just comes back to positioning and situational awareness. You need to know where your team members are and where the enemy is so you can get to the best position to AoE while minimizing the risk to yourself. (I'm still working on this myself)
Yes, kiting is important, but you can only kite for so long without support and then you become a liability because your team is constantly having to pull back to help you. Of course, with how minis are and some classes play, it's easy for some to breach the frontline with relative ease and force the caster into a 2 and 3 vs. situation, that will happen at times, especially when it's personal with some people.
The best advice I have is that you just use your positioning and awareness to make it more effort than what most people are willing to invest into finishing you, once you get a reputation like you'll see players chase you off, but not always try to finish you, because it means over extending past a reasonable limit and putting yourself at risk.
Demo is definitely the strongest untouched class in the game and has been since 1.3, and yes, sins and rangers, and most soldiers can blow you up in a combo or two, but that's why pvp is a group based game. You need to rely on your team and yourself to limit the possibility and risk of those situations, you can't Rambo out on a clothie and expect good results.