As a caster, you don't rely much on armor / protection to survive (or very few), you rely mostly on your position.
Check your position constantly:
- in Jebbal Sag, try to stay in group with at least 2-3 more players. Check constantly the map to know where your group members are positioned, you should never be alone in a zone of the map. Your group members are your survivability, you are their DPS (which means that in a gank you will get most of the kills if you are well positioned).
- in 6 men maps, try to use positions from which you can nuke and where melees can't suddenly"jump" on you (upper platforms for example). Check melees positions, and try eventually to understand if they are moving to you. You should be in a position from which you can nuke and enemy players can't even see you. Or at least are not motivated to reach you.
Other suggestions:
- Use hide to check room and other spaces before enter.
- Don't stand still: move, kite, jump around and try not to be in the middle of a room when your group is ganking. If you play demo, Hands of the Underworld (speedbuff) is the best survivability tool in many situations. If you get chased 1 vs 1 by tanks, sins or other good melees (hard to DPS down), just CC and run away to your group or a safe spot, don't try to kill them.
- If you play a demo, the key spell you have to survive to a melee in 1 vs 1 in a close range is Waves of Flame (you need Rings of Fire class perk) + CCs. Watch players CC resistances, and test his protection. If you have no chance to kill him in a sequence CC + WoF + CC + WoF, run away fast and don't fight him.
Waves of Flames hits really hard, but don't be tempted to use it too much, or you will die more than kill.
You should not try to go close range, but if someone comes melee range, use it to DPS him down (if you can) or run.
I have not been playing demo for long time yet (I'm PvP 6).
Many experienced demos / necros can help you better for sure, but since you haven't got many suggestions so far, I think these are correct and a good start point