Warlock magic is very very powerful. They can kill a full screen of monster with single attack
but they have quite a good cast delay with them for skills.
Here is detail information of skills
Comet is your most powerful skill but has a huge cast delay so be careful when to use it and the monsters in the center of comet will take more damage then the monsters to the end.
leveling is easier here though
he best place to level will be Cursed Abbey 03 killing Necromancers will be fun but can get u killed also
just be careful of the Banshee here they wont die easy with magic
if u happen to get a single necromancer then try Tetra Vortex it will surely kill it if u have decent enough equips
always have amplify magic. and Recognized Spell on for max damage use the skills Crimson Rock and Jack frost they will come very handy here.
Other places if you want to try then Try Ice dungeon with Chain lighting. its also a very deadly attack for water property monsters :)
If you want to go in the new world then party is recommended don't venture alone there.
and as usual keep some wings and pots with you they always come handy in situation of trouble.