DISCLAIMER: I know the backlash I will get, as I can't name half the new abilities since the last update, having no high level warlock. Thus, this is a simple, basic guide for your average warlock. This follows basic tips, and will be in essay form, not for specific classes. This is SOLELY opinion; if you'd like to add something, write your own guide.
Warlocks are an excellent class for PvE and takes strategy, skill, and zero percent mistake rate to be successful. A Warlock thrives from using his demon and DOTs. In essence, a Warlock is a spellcaster counterpart to hunters when being attacked; ignore the demon, just kill the warlock. You might want to take up engineering for the useful bits such as the Net-O-Matic and bombs with stun effects. Fear is, by far, a Warlock's best friend. The only downside of fear is that you'll have to pursue your target to land good DOTs such as Immolate and Corruption. A good counter for this downside is to try and Fear them into a wall or tree. Then you can cast spells freely without moving.
Another great spell is Curse of Tongues. This is your best weapons against casters, as it increases casting time by about 50%. Another problem is Healthstones. They are an excellent alternative to Potions, but as they do not stack, you'll run out of room quickly. Always carry a soulbag, and buy one at first opportunity. You can regain the lost cash easily. Banish is a great spell when fighting hunters, other warlocks, or multiple enemies. It takes out an enemy out of the battle for 30 seconds. If you're fighting multiple enemies, Banish the healer.
Moving on to demons. Despite the Imp's small stature, it can prove quite an asset. It can give a fire shield, can add stamina, and proves as a useful subnuker with the low-end Firebolt spell. Voidwalker, more or less, are the worst choices for PvP. It's only good for a beefbag, and even then they are quite useless. Despite it's taunt spell, it's common sense to ignore the Voidwalker and kill the Warlock with ease. The Succubus, the third demon, is probably the best choice for PvP and, in some cases, PvE. Its Seduce spell, which causes hibernation for 10 seconds, is very useful when engaged by a tank or a rogue. Speaking of rogues, it can be good at ambushing as well, due to its stealth ability. People will be in for quite a surprise when attacked by a stealthed Succubus.
When in PvP, I always have a Succubus with me(undead warlock, level 25) . If I'm suddely attacked, just Seduce them right of. If it's a caster, I just use Curse of Tongues. During a standard magic battle, there is a simple combination I use (and, if needed, mana potions included). Immolate, Corruption, Curse of Agony(non caster only, replaced by Curse of Tongues if caster), Shadow Bolt, Shadow Bolt, SHadow Bolt, Fear, Shadow Bolt, Shadowbolt, Net-O-Matic/bombs. (Only use the last part if available or if the situation dictates.)
Due to the poor quality of my guides (e.g. Paladin and Warrior guide), please include any missing skills or removed skills. If you'd like to add your own strategies, PM me, or better yet, write your own guide.
CHEERS!
~TheUltimateUndead