"DING! Level 25! Finally, I get to go to the Abyss! I can't wait to fly around in there, find some enemies, and smack them dead! Oh, and I'm going to go take some artifacts and a fortress while I'm at it! I'm going to own every..."
"What the...? I died? What the heck killed me? Oh, there's a Sorcerer behind me laughing. Did he just one-shot me? How am I supposed to fight that?"
I, like a number of you no doubt, went to the Abyss and quickly learned the hard way that it is very, very unfriendly to a level 25. I wasn't aware of some of the PvP mechanics, such as higher level players getting bonuses to defensive stats such as Evasion and Magical Resist, which makes them very difficult to hurt. And then there's the fact that they often band together in roving gank squads and hunt down lower level players and kill them for 1 XP, 1 AP, and 1 Lulz. It's possible to do your own thing in these areas, but it's not easy. Grouping helps, but isn't always viable. Plus, PvP happens outside the Abyss too thanks to rifts. Here's some general advice on how to survive a PvP encounter. Not necessarily to win it, just come out of it alive. If you're strong enough to defeat your foe, then by all means do it. But, sometimes, that's not an option.
One of the best tricks you can use to avoid being caught off guard by the other faction is to create a sort of early warning system. This is actually rather easy to do: simply create a chat tab and set it to display enemy combat spam. All of it. You'll want any warning you can get, especially when it announces an Assassin used hide. I personally set the text to some nice red, yellow, and orange colors. I did multiple colors so I'm more likely to notice it scrolling the combat spam. I also went ahead and put enemy death in there as a bright green color so I know when an enemy has died (by default, ally and enemy PvP deaths both appear white, which is confusing). Once you have your tab, put it somewhere you can see it easily while going about your business.
This chat tab provides two very good advantages: First, combat spam has a longer range than your radar. So if xxxgankerxxx is attacking iracarebear somewhere off your radar, you'll see it on your chat tab. You won't know which specific direction it's in, but you'll know the enemy is near and that you can be ready. The second, major advantage is that the combat spam will also give you the name and rank of the skills the enemy is using. This is valuable, as it'll give you an idea of the level of the foes in question. The level difference between, say, Swift Edge II and Swift Edge IV is huge. In general, if the ranks of the basic skills being used are higher than the ranks of yours, then the enemy is probably a higher level, which in turn means they'll have a big advantage over you.
Second is to keep an eye on your radar when you're in the Abyss or a PvPvE zone. Any enemy not in a hidden state will appear on your radar, even if there's a big altitude difference.
If you look around and stay alert, you may spot them before they reach you or, more importantly, before they spot you. Avoiding getting jumped will greatly improve your survival rating. Even if you could kill the opponent normally, if they jump you, they may kill you instead! Also, unless you have a visual of their character, always assume if you see a big red dot on your radar, they see a big red dot on theirs. And remember, you can set a key that will automatically rotate your camera 180 degrees without changing the direction you're going.
Third is to run away. But just peeling off and running as fast as you can won't necessarily work. You need to out run your enemy, need to mitigate any ranged damage they have, and, most importantly, have a way do deal with any mez effects such as sleeps, stuns, and roots, that they may have. Tossing on whatever protective abilities you have as well as speed boosts will help. Assassins and Rangers can simply hit hide. And consumables, which I'll cover in a moment, can handle much of the rest. However, even still, just running away can hurt you more than standing. Why? Running forward lowers your defensive attributes! Running backwards boosts physical defense and parry (I think movement might be a little slower though), whereas sideways boosts Evasion. Both lower attack power, but that's not important during retreat. Of course sideways and backwards may make it hard to see where you're going, but that can be overcome by left-click rotating the camera. The direction you see will change, but the direction you move won't.