There arent any superquick ways to level, but you have many choices to level reasonably fast, and can pick one suitable for your character. Generaly you need to kill monsters level 11+ in large quantieties. So you can go to Port Malan and kill all bearguys there on surface and in 2 caves, go to Whitetower and kill icewolves in wyvern mountains and theres also a cave there , go to Freetown and keep killing bandits around there , or equip fire weapons and clear Trollfens + troll cavern , Go to Fogas and farm Abbey of saint Admus, or echos cave for vorators , or Iross highlands for werevolves (thats mostly available for mages because they have pure fire weapons) or go to Lamis and farm Forbidden pit (that one is good for warriors with adamantite swords) . And there is also minotaur maze and ashen minotaur temple when you unlock it. And also snakes ,apes and rexodons, just you actualy cant farm them yet
There are many choices and to not be bored you can hunt rumour boses and kill things around the place where they are found.
If you just want to farm exp fastest ,choose a place that you can clear fast without using lot of potions, and thats it.